Those are clever tricks. Obvious when you think about it.
Steve Crane
about.me/stevecrane
On Sat, 10 Apr 2021 at 15:10, Scott Ransom <mr.ransom@xxxxxxxxx> wrote:
The other truth that I learned (but a bit longer ago) about percentages
that I really wish they had taught me in school is that X% of Y *is the
same* as Y% of X.
So if you're confronted with 12% of 25, you can just turn that around and
calculate 25% of 12. Which I can calculate in my head almost instantly as
3 (which is the correct answer to the problem no matter which way it's
stated).
12% of 25 = 25% of 12 = 3
This ability to turn them around doesn't always make a percent easy to
calculate in your head, but it sure helps in some cases. But was I ever
taught this in school? Nope. Had to wait until they developed YouTube.
Scott
On Sat, Apr 10, 2021 at 5:34 AM Spring (Stacy) Dew <springdew@xxxxxxxxx>
wrote:
Because New Math. Because Common Core. Because every time somebody wants
to teach multiple approaches to math, a gang of parents and grandparents
who didn't learn it that way bludgeons then into submission.
On Sat, Apr 10, 2021, 7:29 AM Scott Ransom <mr.ransom@xxxxxxxxx> wrote:
I hate learning arithmetical tricks that I should have been taught in
school and wasn't. Like, these tricks aren't hard, and they're kind of
obvious once you think about them, but unless you deconstruct how
arithmetic works you may not realize they're even there. Stuff like the
below should be taught in schools.
I learned that to take a percentage of something, I just have to do the
multiplication. 16% of 410 mean I need to multiply 410 by 0.16. And that
probably will require paper and pencil or a calculator if I want to get it
right, because I can't do 410 x 0.16 that easily in my head.
Except that I can. I saw a video that laid out some hard truths about
percentages that I probably would have thought of if I'd ever thought about
it, but didn't, and so should have been taught explicitly.
Percentages are additive.
And yes they are, but unless you're explicitly taught that, you may not
realize it.
You can look at 16% of 410 as requiring you to multiply 410 by 0.16,
which I can't easily do in my head. That's the technique I learned in
school.
OR you can look at it as taking 10% of 410 (easily calculable in my head
as 41), then taking half of that (i.e., 5%, in this case 20.5, also easily
calculable in my head) and then taking 1% of 410 (also easily calculable in
my head as 4.1) and adding them together.
41 10%
20.5 5%
4.1 1%
----- -----
65.6 16%
And that's pretty easy to do in my head.
Why is this not taught in school? I get it's obvious if you are the
sort that deconstructs math, but most people don't do that. They just work
the techniques they learned in school. And this isn't one of those
techniques, or at least wasn't when I went to school. But it seems like it
would be *quite *useful to have known. It makes calculating 51% of
things a heck of a lot easier than it is using the standard "multiply it by
0.51" method - calculate 50% (easy), calculate 1% (easy), and add.
Duh.
Scott
