The quickest way to calculate percentages is to multiply numbers first and handle two decimal places later.

For example, 20% of 70 is 20 x 70 = 1400, so the answer is 14.

70% of 20 is also 14.

To calculate the percentage of 72 or 29, then round up and down to the nearest multiple (70 and 30 respectively) to get a quick approx result.