To increase an amount by 28%, multiply by

To increase a number x by 28%, you do the following:

f (x) = 1.28x

Increase by 28% means original+28% of original

28% of original is 0.28 times original

so

original+0.28 times original=1 times original+0.28 times original

undistribute original

original (1+0.28)

original (1.28)

so multiply it by 1.28 to incrase by 28%