20% of what number is 18
explain.

In this question, the thing that needs to be found is the number whose 20% would be 18. The information's required for getting to the solution are already provided in the question. based on those given information's, the answer can be deduced very easily.
Let us assume the unknown number to be = x
Then
(20/100) * x = 18
x/5 = 18
x = 18 * 5
   = 90
so the unknown number in the question is 90. I hope the procedure is clear enough for you to understand.