Question

To increase an amount by 28%, multiply by

Answer 1

To increase a number x by 28%, you do the following:
f(x) = 1.28x

Answer 2

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%