Yes. Being a smart consumer requires at least some basic math, like interpreting percent (and also the aforementioned unit pricing). And when observing politicians’ promises, we all need to know ‘how to lie with statistics’ and how to detect when someone is trying to do that to us.
Combinations and permutations has occasionally been useful (for me it was in the context of planning activities for students, or at an upcoming party).
No current plans to build it, but I was thinking foam filled limbs with fiberglass skins.
Cool. I hope there will be video !
During an SAT prep class I took 45 years ago, they drilled us on knowing the rough fractional equivalents of percentages (eg, 1/7 = ~14%) which has been pretty useful to have in easy recall.
Oh, yes: likely every day for people who look at prices or recipes or home-improvement topics or news stories (comparing national stats on medical or economic issues, for instance). Or at a lot of other things.
Rote memorization does have its uses!
Since no one else has asked it, I will. Just what do you mean by cross multiplication. To me, it means vector cross product, but I rather doubt that is what you mean.
IRL, I have used the quadratic formula a number of times.
It is actually trigonometry, but I was once visiting a botanical with my and his SIL, who was a rising HS senior. We wanted to estimate the height of a tree. So my son got down on the ground and I moved to where he could see the top of my head and the top of the tree in a line. We then paced off the distance between me and my son and me and the tree. Knowing my height gave us an easy ratio for the height of the tree. SIL was totally mystified by this.
\frac{a}{b}=\frac{c}{d}
ad = bc
Yes-- but you actually can give “110%.” It just means you are giving more than you gave the last time. As a metaphor for playing a sport, or some other task, assuming you improve the more you do it, you may have given “everything,” ie, 100% of what you had, last time, but this time you have more to give. So if you could quantify your enthusiasm and skill, you have now “110%” compared to what you had the last time.
And warship engines can give 110% as well - where 100% is the maximum for safe engine operations (presumably when the Captain gives that order, he’s aware of conditions that make a bit of extra stress on the engines the least of his problems)
Heck, my Nissan Leaf can be technically charged above 100%, because it’s still pretty new, and in warm weather will charge over its spec.
It is overengineered, because the warranty says the battery will hold a full charge of “150 miles,” and that warranty pretty much replaces any failed part for free in the first 3 years, needs to charge to 180 miles in the first year to still be charging 150 miles by the 3rd year.
If it’s charging 140 in the 3rd year, you will be asking to have the VERY expensive battery replaced before the warranty expires.
My plan is to trade my car for new if it is charging right around 150 all the time; get the battery replaced if it drops below 150 during the warranty period, then plan on driving it about 2 more years; if it still charges 165-180, which it does now, I will buy the extended warranty and keep it.
All about that battery.
I interpret it to mean “Give it all you’ve got, and then some.” Which you can’t literally do; but it’s not meant to be taken literally. It’s hyperbole, like asking for something to be done quickly by demanding “I want it yesterday!”
I use it for solving for X. In my head, I’ll say something like “3 goes with 8 like 15 goes with X. And then write it out:
\frac{3}{8}=\frac{15}{x}
Then I cross multiply to get
3X = 120
And solve for X.
That’s pretty fuckin funny! 
Whereas I would probably do that in my head by thinking “15 is five times as big as 3, so what’s five times as big as 8? 40.”
I would too with the numbers I used as examples. But when it’s 4.98, 28.67, and 874, it gets a lot harder to do in your head.
Yeah, I almost added “…but that works because the numbers are nice.”
You got your decimal point right. = )
People are really that bad at simple arithmetic?
The fun part is when people estimate how many square feet in a square yard… Three feet in a yard - so three times as many?
What does sadden me some, though, is when I see students solving a problem like \frac{8}{3} = \frac{x}{15} by first cross-multiplying to 120=3x and then dividing by 3. You just added two extra steps. Cross-multiplying is often useful, but you need to understand why it works, and that it’s actually two steps, each of which is also valid by itself.
Well, I wouldn’t want to make you sad, so how do you do it in 2 fewer steps?