[bigger_cheese]

I don't know if this is a trick but the fast way I did that problem quickly in my head is 20% = (10% X 2) i.e calc 10% of the number then double it.

To quickly calc 10% just multiply the number by 0.1 which you can do by moving the decimal point one place 20,000.00 => 2,000.000 then it is easy to double that number.

to get 4,000.

17% for example is 1.7 x 10%

in this case 1.7 x 2,000 = 3,400

[skydhash]

For me, it's just that 20% is one fifth. One fifth of 20 is 4 and you add the remaining zeroes.

You mostly have common equivalences like this in your memory and you can be faster than computing the actual thing with arithmetic. Or have good approximations.