Question

55 is 40% of what number

Answer 1

It is important to do the calculations very carefully. Otherwise there is nothing complicated about this problem.
Let us assume the unknown number = x
Then
40x/100 = 55
40x = 55 * 100
40x = 5500
x = 5500/40
  = 550/4
  = 137.5
So 55 is 40% of 137.50. I hope the procedure is clear to you and in future you can solve such problems on your own without requiring any help from outside.This is the simplest way of solving such problems.

Answer 2

55 is 40% of 137.5

Convert your percentage into a decimal:
40%/100 = 0.4

Divide 55 by the decimal: 
55 ÷ 0.4 = 137.5