I believe the answer is 25. If "of" means multiply, 80% * x is 100. 80% is equal .80. Since we want to find the number(x) is, we will get x alone. So the opposite of multiplication is division. We divide 100 by .80 and we get 125. We could check this answer by multiplying 125*.80 and we get 100. So the number is 125 but what is 25% of it. .20 * 125 and we get 25. To check, we could say that 25/125 is .20.
