It's not a "mental math trick", it's a straightforward calculation you should be able to do in your head.
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
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.