&quot;Simple high school math&quot; which is sometimes taught in a single hour of a single day and forgotten about by tomorrow because it isn&#x27;t on the test and we really need to cover the quadratic formula before the mid-term.<p>Something that I saw on HN not too long ago that really benefited me was something I <i>should</i> 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.<p>What is 36% of 25? I&#x27;ll be honest. I find that&#x27;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&#x27;d estimate it to be a bit over 8. Honest guess.<p>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.<p>Now the &quot;I&#x27;m an idiot&quot; 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&#x27;t. :)<p>Given the popularity and support the tip received - I&#x27;m going to guess I&#x27;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&#x27;t <i>think about the math</i> in decimal form. So they miss out on something that is obvious once someone stops and points it out.<p>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&#x27;m also an average person.)