Let us assume the unknown number in the question = x
Then
64/100) * x = 80
64x = 80 * 100
64x = 8000
x = 8000/64
= 125
From the above deduction we can already see that the unknown number in the question is 125. This is the simplest way to get to the answer for this type of problems. Only be careful about the calculation part. Especially the multiplication part. Other than that there is nothing tough in this problem.