Simple maths

 Far too many people in the world have no idea what to do if you ask them what's fifty-three times forty-seven. Anyone who can't say "two thousand, four hundred and ninety-one" after a couple of seconds' thought is missing the very basic trick that can make people think you're some kind of genius.

Everyone probably did learn at school that (x+y)(x-y)=x²-y², but they don't know that it has practical applications far beyond getting the right answer in an algebra test and then forgetting it forever. It means that if you want to multiply something that's a bit more than a nice round number, by something that's the same bit less than that nice round number, the answer is the nice round number times the nice round number, minus the bit times the bit.

So while 53x47 sounds like you'd have to get a pencil and paper and try to remember how to do long multiplication, it's actually just 3 times 3 (which everyone knows) less than 50 times 50 (which nearly everyone knows; but if you don't, it's just five times five and then add the two noughts on to the end). So it's 2500 minus 9, and it's not too much of a strain to see that that makes 2491.

Try it with other numbers, and check it on your calculator if you don't believe me. Then you'll be able to show off the next time a strange man is roaming the streets asking "What's seventy-two times sixty-eight?" and jeering at anyone who doesn't know.