No, the joke is “In my day it was horses and trains and now it’s electric cars.”
Maybe the two sisters are not familial “sisters,” but rather nuns. Ah-ha! What if Jim lives in a multiparent (as in more than two) family?
Good ones!
While technically true, the issue with this problem is that it is unclear enough that multiple incompatible assumptions can be made, as different people have shown in this thread. The question is unclear.
It’s really not. Those people are being silly.
Multiple incompatible assumptions always can be made if you try hard enough. But there’s one set of obvious and simplifying set of assumptions that is clearly correct.
Again: it’s a math word problem not an exercise in probing the depths of the ambiguities of English. Try less hard to fail to understand it and it’s very clear.
I don’t agree. I definitely read it as starting at 85% full, saying she’d already charged it that much. Yet others read it as how long it takes to charge to 85%.
The word problems I remember in school were a lot more clear.
As long as we’re being pedantic, I’ll point out that we don’t know whether or not we are given the whole of the problem, or whether there’s more to it than just the part the father reads in panel 2.
85% is definitely the final state of the battery, because she’s charging it to that. But we don’t know the initial state of the battery, and that’s essential. And there’s no likely and obvious assumption for the initial state of the battery: The only obvious assumption is that it starts at 0%, but that’s very unlikely. It’s much more likely to start at somewhere between 15% (the recommended minimum) and 50% (because most people won’t charge something that’s already over half), but no single value in that range is obvious.
Did you read it that way initially and realize that you were mistaken because it required some careful parsing, or do you still read it that way? I can see making that mistake in a quick reading, but on reflection I don’t know how to parse “If Sonia charges her EV to 85%” as “The EV starts at 85% and goes up from there”, but perhaps I’m just not thinking about the many ways that English is confusing. Can you give me an example of another sentence where the word “to” means “starting at, and continuing onward from”?
Only if you think that a word problem has to map exactly to a real-world system. But it obviously doesn’t, because real-world charging is not linear or 100% efficiency. No physical system is perfectly linear or without epicycles. But we can still write and answer word problems based on simple models.
I think even if you really want to claim that there’s no reasonable assumption for the initial state of the battery, then the question still has a clear and well-formed answer. Instead of saying “four hours and whatever”, the answer is “At most four hours and whatever”. Word problems can include inequalities, and whether one is likely to is going to depend on the context provided in the class.
But I’ll go back to my baseball word problem analogy. Focusing on how much charge the battery has initially is exactly like focusing on how much money Jim already has in his pocket. It’s completely missing the point in favor of irrelevant minutia.
The initial charge is the one piece of information that is necessary for the problem that is not provided. “How long to fill a five gallon bucket…” it’s reasonable to assume the bucket is empty when starting, but assuming the battery is empty is not a reasonable assumption. Buckets are empty before filling them all the time, but EV batteries being completely empty is a very rare event.
The only gotcha with the baseball question I see is the assumption Jim and his sisters are children. Taxes can be ignored, the same as charging efficiency and voltage drop, unless the question is in a context specifically designed to test those concepts (“for the following questions sales tax is 8.25%” “quote the price of installing a circuit for a customer who must charge their car in 4 hours”).
People have some money in their pockets quite regularly too, but it’s just as silly to say that we can’t answer the question because we don’t know how much money is in Jim’s pocket.
Because the charge of the battery is not provided, there is exactly one reasonable assumption, and it’s the correct one.
If any student presented this as an unsolvable problem, the correct response would be “you’re very clever to have noticed that, but please spend more time trying to answer the problem given a charitable reading of it and less time trying to not answer the problem.”
If I had a word problem that didn’t specify how much money someone had in their pocket when that was relevant, I’d levy the same criticism against that word problem. Insufficient information is insufficient information.
The question was how much money does Jim need, not how much more money does Jim need. If the question was “how much does Jim need to ask his parents for” or “how much does he need to get from the ATM” (trick question, the hot dog vendor accepts credit cards!), then how much he had to start would be relevant, but as asked, I interpreted the desired answer as how much does he need to hand over at the concession stand, not where does the money come from.
Charge is different 0% charge is a very bad assumption, because unless the question was “after having to push the car the last 12 feet to reach the charger…” the car is not at 0% (I know EVs will show 0% and still have several miles of range, until they don’t).
0% is a poor assumption, just as 84% starting charge would be a poor assumption. As said, anything between about 50% and 5% is reasonable for an everyday starting pre-charge state. If grandma is at the range limit of an 85% charge, then anything between 80% and 5% is a reasonable assumption.
Some real-world observations:
- The charging rates quoted in the problem are realistic and typical of single-phase AC home charge points. I have a 32A charge point which is on a 40A breaker and 16 square cable.
- While the 240V 30A charge point in the question works out at 7.2 kW, the actual charge rate is limited by the car’s onboard AC charger, which is quoted as 6.6 kW for my Leaf.
- As others have noted, you will never be starting with zero state of charge.
- With AC chargers it is reasonable to assume constant charge rate up to (very near) 100%. The issue of charging slowing down only applies to fast charges (e.g. 50 kW).
- There is no problem with charging the car battery to 100% using a slow charger. There is advice not to leave it at 100% for a long period.
The question is how long does it take the battery to charge to 85%, not how much more time it needs to charge (from wherever it is).
OK, then, here’s another word problem: How long does it take Jim to reach Schenectady, traveling at 65 MPH?
My word problem is pretty analogous to the one in the OP. Yours isn’t.
But I think we agree that yours is poorly written.
It might be possible to write an analogous one that we both agree is unclear. Would you like to try?
Were I to get that question in class as a kid, I would assume Chicago (where I live) as a starting point. Otherwise, there is no answer. I would probably write “not enough information given, but assuming here as a starting point.”
Yea, it can be frustrating and stupid. I do find myself with my third grade daughter sometimes casting aside my irritation at a poorly written question, but I can figure out well enough what the question is getting at.
This question sucks. I’m okay with assuming starting at zero percent. I’m okay with assuming a constant charge.
But where the hell is Grandma’s house? I guess we are supposed to assume that the 85% charge is what is needed to get to Grandma’s house, but then the question would have been much clearer if it was worded like this:
“Sonia wants to go to her Grandma’s house. She needs an 85% charge in her car before she starts the trip. If her battery has 40kWh of capacity and is at 10%, how long will it take before she can leave for Grandma’s place after she plugs into a 240V, 30A charger?”
I used to work for a company that created exam prep materials for high schoolers. We hired college students to write the questions, then published them in books. We had to reject an awful lot of them for being terribly written. I don’t know
why question writing is so hard for many people.
It doesn’t matter. But you’ve reworded it as I would have to make it clearer, including the starting charge. I would have been fine with 0% starting charge, even if it isn’t a real-world or likely real-world condition (which I didn’t know, nor care about, as this is just a word problem.)
But, yes, everything about your re-write is better.