'is' in mathematics means equal
'of' in mathematics means multiplication
If we translate "What number is 85% of 60?" into a mathematical equation, we get:
x = 85% * 60
85% can also be expressed as 85/100, since % or percent means over 100.
x = 85/100 * 60
x = 0.85 * 60
x = 51
51 is 85% of 60.