'is' in mathematics means equal
'of' in mathematics means multiplication
If we translate "16 is 25% of what number" into a mathematical equation, we get:
16 = 25% x
25% can also be expressed as 25/100, since % or percent means over 100.
16 = 25/100 x
16 = 0.25 * x
x = 16/0.25
x = 64
16 is 25% of 64.
