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