Know 10%, 5%, and 1%, and you can
calculate any percentage mentally

17% may look difficult to calculate mentally, but split it into 10% + 5% + 1% + 1%, and each part is easy. For 10%, move the decimal point one place to the left; 5% is half of 10%; and for 1%, move it two places to the left. Change the percentage and amount with the sliders and see how it works.