Comment by DanBC | Hacker News Reader

Comment by DanBC | Hacker News Reader

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You accept you might make a silly mistake while doing the calculation in your head. How do you know you won't make a silly mistake when you do the calculation using a machine?

If you do both, or at least estimate the result, you can check the results.

I don't think you'd make a silly mistake when using a calculator, but I think other people might.

I think (although I have nothing to support me) that most people cannot use percentages in any meaningful way. If you say "I have a tv that currently costs $300, and I'm going to give you a 10% discount. What will it cost after the discount?" then they can show you the buttons they'd push to get that result. But if you ask them a slightly different version "I have a washing machine that currently costs $800. I've already given you a 10% discount. How much did it cost before the discount?" then I think you'll find a bunch of people who don't know what buttons to push. (Or worse don't know that they don't know and who'll get a close but incorrect answer.)

So, that's not the kind of mental math trick talked about in the article, but calculators are tools and tools are most useful to people who know how to use them and most calculators and calculator apps don't make it easy for people who don't know math.

What you are describing is high school math. Really simple high school math. Maybe even junior high school math.

I would consider a trick something like, multiply large number by really large number in your head. That is a waste of time.

"Simple high school math" which is sometimes taught in a single hour of a single day and forgotten about by tomorrow because it isn't on the test and we really need to cover the quadratic formula before the mid-term.

Something that I saw on HN not too long ago that really benefited me was something I should have been taught in school. I probably was - but as I mentioned. It was taught in a rush and I had forgotten it, if it was even taught at all.

What is 36% of 25? I'll be honest. I find that's a pretty tricky one to do in my head. I can estimate it to 8 by taking 33% of 24 (close enough, right?). So I'd estimate it to be a bit over 8. Honest guess.

Well the trick I learned on HN is to reverse it. Since % is really multiplication, the commutative property applies. Take 25% of 36 instead and bam! The answer is a flat 9. No estimating needed.

Now the "I'm an idiot" part comes from the fact I always did percentage as multiplication of a decimal. I should have intuitively put 2 and 2 together and figured out the commutative property applies. I hadn't. :)

Given the popularity and support the tip received - I'm going to guess I'm a good example of the average person. The average person was taught percentages in a way that they can convert to decimals but don't think about the math in decimal form. So they miss out on something that is obvious once someone stops and points it out.

So to cut my rambling short: simple math is rarely simple for the average person. The average person, from my experience, is worse at math than they would probably like to admit. (And I'm also an average person.)

Cute trick, but what is 36 * 25? Why is the problem harder when you take out the most trivial part?

It's harder because people fail to make relations or don't think about how they can simplify a problem. Or big numbers are generally harder for mental math. Either or.

Solving 25% of 36 then multiplying by 100 (effectively removing the "%" part of 25%) gets 900. It's also a bit indirect, so many people would overlook it (myself included).

If you were to ask me what 36 * 25 was in most any other context I'd do 40 * 25 = 1,000 - 4*25 = 900.

Although, thanks to your prompt, I'm likely to remember to see if I can cheat with percentages. :) Smaller numbers and fractions are more intuitive for me.

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