# Is this math story problem in Baby Blues legitimate? **URL:** \ **Category:** Factual Questions\ **Created:** [October 27, 2022, 2:10am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849 "2022-10-27T02:10:37Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [October 27, 2022, 2:10am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/1 "2022-10-27T02:10:37Z")
The [October 26, 2022 _Baby Blues_](https://www.gocomics.com/babyblues/2022/10/26) strip has this math word problem: > If Sonia charges her EV to 85% at a 240v, 30 amp source, and her car’s battery has a capacity of 40 kwh, how long will it take before she can drive to her grandma’s house? Darryl observed that he and Wanda are dinosaurs, to which observation Wanda added, “And the comet is coming”. As for me, that question makes me feel like Mr. Wilson did when he observed Dennis, Margaret, and their classmates in a math class dancing to some pop music. Anyway, is the \*Baby Blues" math story problem a legitimate one and, if so, what is the answer and how does one arrive at the answer? ---
**Author:** ![dolphinboy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dolphinboy/32/330_2.png) [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)\ **Post date:** [October 27, 2022, 2:13am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/2 "2022-10-27T02:13:42Z")
Nevermind ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 27, 2022, 2:14am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/3 "2022-10-27T02:14:07Z")
No, it’s impossible to answer. One would need to know the distance to grandma’s house, and the miles/kwh of the EV so solve. ---
**Author:** ![Tabco](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tabco/32/7644_2.png) [@Tabco](https://boards.straightdope.com/u/Tabco)\ **Post date:** [October 27, 2022, 2:15am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/4 "2022-10-27T02:15:57Z")
I was wondering if it was assumed that 85% charge is what was needed to get to grandma’s house, so the focus was on time needed to charge to 85%. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 27, 2022, 2:20am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/5 "2022-10-27T02:20:02Z")
If we assume that she leaves when the car is fully charged, and assume that the charging unit is completely efficient, then it is possible to solve. The amount her battery needs to be charged is 40kwh\*(1-0.85) = 6 kwh. The charging rate is 240v\*30amp = 7,200W so 6kwh/7.2kw = (1/1.2) hours = (60/1.2) minutes = 50 minutes. ETA: whoops misread the question, if she needs to charge it by 85% then 40kwh\*(0.85) = 34 kwh. The charging rate is 240v\*30amp = 7,200W so 34kwh/7.2kw = 4.7222 hours = about 4 hours and 43 minutes and 20 seconds. ---
**Author:** ![Hilarity\_N.Suze](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hilarity_n.suze/32/6223_2.png) [@Hilarity\_N.Suze](https://boards.straightdope.com/u/Hilarity_N.Suze)\ **Post date:** [October 27, 2022, 2:27am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/6 "2022-10-27T02:27:10Z")
Well, and how old is Sonia? If she’s 14 it will take at least two years no matter how fast her ev charges. ---
**Author:** ![ekedolphin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ekedolphin/32/10003_2.png) [@ekedolphin](https://boards.straightdope.com/u/ekedolphin)\ **Post date:** [October 27, 2022, 2:27am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/7 "2022-10-27T02:27:16Z")
I realize there wasn’t enough space to write “asteroid”, but comets and asteroids are two different things. ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [October 27, 2022, 2:30am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/8 "2022-10-27T02:30:19Z")
> [@Tabco](#): > > I was wondering if it was assumed that 85% charge is what was needed to get to grandma’s house, so the focus was on time needed to charge to 85%. I was focusing on 85% being that’s what Sonia considers good enough because she doesn’t have time to charge it all the way. > [@Buck\_Godot](#): > > If we assume that she leaves when the car is fully charged, and assume that the charging unit is completely efficient, then it is possible to solve. > > The amount her battery needs to be charged is 40kwh\*(1-0.85) = 6 kwh. The charging rate is 240v\*30amp = 7,200W > > so 6kwh/7.2kw = (1/1.2) hours = (60/1.2) minutes = 50 minutes. > > ETA: whoops misread the question, if she needs to charge it by 85% then > > 40kwh\*(0.85) = 34 kwh. The charging rate is 240v\*30amp = 7,200W > > so 34kwh/7.2kw = 4.7222 hours = about 4 hours and 43 minutes and 20 seconds. Thanks! I’m definitely sharing this one with the Math teachers in my office. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 27, 2022, 2:36am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/9 "2022-10-27T02:36:20Z")
> [@Monty](#): > > I was focusing on 85% being that’s what Sonia considers good enough because she doesn’t have time to charge it all the way. Yes that was clearly what was intended as I said I misread it the first time. I’m often pleasantly surprised at how good comic strips are at presenting realistic math. FoxTrot was particularly adept at this, including real math up through calculus. ---
**Author:** ![Folly](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folly/32/3219_2.png) [@Folly](https://boards.straightdope.com/u/Folly)\ **Post date:** [October 27, 2022, 2:41am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/10 "2022-10-27T02:41:32Z")
> [@ekedolphin](#): > > I realize there wasn’t enough space to write “asteroid”, but comets and asteroids are two different things. Recent(ish) theories speculate that it is more likely to have been a large chunk broken off of an even bigger comet that broke into many pieces as it approached the sun. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 27, 2022, 2:41am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/11 "2022-10-27T02:41:46Z")
That’s a whole lot of assumptions. What is the initial state-of-charge? ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [October 27, 2022, 2:43am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/12 "2022-10-27T02:43:00Z")
Anyone remember which of the _Dennis the Menace_ episodes had the scene I described? That was obviously a real math story problem as Dennis solved it, explaining quite clearly to Mr. Wilson how to solve it. At the moment searching the Internet is a hit/miss (mostly miss) thing because the 20th CPC conference only just concluded and it takes a while before we get back to normal after that. ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [October 27, 2022, 2:44am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/13 "2022-10-27T02:44:23Z")
Good point! Shouldn’t that be stipulated in the problem? Perhaps, another assumption, the initial state is zero charge. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 27, 2022, 2:51am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/14 "2022-10-27T02:51:02Z")
I agree its poorly worded and requires a lot of possibly faulty assumptions, but that is not atypical among textbook word problems. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [October 27, 2022, 2:58am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/15 "2022-10-27T02:58:11Z")
Perhaps the best way to answer your question is to say that if you know the capacity of a battery and the rate at which it can be charged you can work out how long it will take to charge as a rough first approximation. In reality the problem has gaps (like the one @beowulff mentions), plus you need to approximate. In reality charging is neither 100% efficient nor necessarily linear. ---
**Author:** ![Seanette](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/seanette/32/9139_2.png) [@Seanette](https://boards.straightdope.com/u/Seanette)\ **Post date:** [October 27, 2022, 3:00am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/16 "2022-10-27T03:00:39Z")
It’s been a while since I read Baby Blues, but aren’t the kids in that strip a little young for this type of math problem? Ohm’s Law even got in there (at least as far as I can tell, what with converting volts and amps to watts to kw/h). ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [October 27, 2022, 3:09am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/17 "2022-10-27T03:09:21Z")
I’m assuming the humor the author was going for is that kids that age are far more advance academically than their parents were at that age. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 27, 2022, 3:18am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/18 "2022-10-27T03:18:20Z")
> [@Monty](#): > > Anyone remember which of the _Dennis the Menace_ episodes had the scene I described? I don’t remember it but I did manage to find it on youtube.(Google search: dennis the menace show math class) [![](https://img.youtube.com/vi/2VTF5L74OHs/hqdefault.jpg "Dennis The Menace - S04E29 - The Three F's") ](https://www.youtube.com/watch?v=2VTF5L74OHs&t=815) ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [October 27, 2022, 4:12am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/19 "2022-10-27T04:12:42Z")
> [@Buck\_Godot](#): > > so 34kwh/7.2kw = 4.7222 hours = about 4 hours and 43 minutes and 20 seconds. So the actual answer is: “4 hours, 43 minutes and 20 seconds… or less…” Charging to 85% is a standard recommendation for EV’s - don’t habitually keep charging to 100%, stop at 80-90% except in special circumstances (like a long trip) and also avoid making it a habit to drain the battery below 10% or so. Even doing this a few times won’t wreck the battery, but doing it all the time will shorten the battery life. Same goes for high-speed supercharging. It doesn’t hurt the battery to do it from time to time, but the less you do so (slow overnight charging at 7.2kW instead) the more life your battery will have. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 27, 2022, 6:33am UTC](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849/20 "2022-10-27T06:33:29Z")
> [@Seanette](#): > > It’s been a while since I read Baby Blues, but aren’t the kids in that strip a little young for this type of math problem? Definitely. She’s only about 8 or 9. You might want to give this to a HS physics class that’s learning the basics of electricity, but not grade school kids. > [@Princhester](#): > > In reality charging is neither 100% efficient nor necessarily linear. AIUI, chargers slow their charging rate as they approach 100%. I always thought this was because it’s harder to stuff more electrons into a battery as it gets full. But I’ve seen comments elsewhere that this is because the chargers don’t want to over charge the battery, since that might damage it. Perhaps both of these are right. [Next page](https://boards.straightdope.com/t/is-this-math-story-problem-in-baby-blues-legitimate/973849.md?page=2)