How Do You Add A Percentage To An Amount?

Multiply the amount by the percentage. Add the product to the original amount.

Example
$400 + 10% (of $400) = $400 + (10%*$400) = $400 + $40 = $440

Percent means "per hundred" (/100). 10% = 10/100 = 1/10 = .1
You can multiply using either the fraction form or the decimal form.

Because percent is "per hundred" or "hundredths", you can put the numerical percentage value in the "hundredths" place relative to the decimal point.
12% = .12 = 12 hundredths = 12/100

This works even with fractional percentages.
37.5% = 37.5/100 = .375
6.25% = 6.25/100 = .0625