There are 25% more black marbles than white marbles in a bag. After Andy removed 40% of the black marbles and increased the number of white marbles by 20%, there are now 36 fewer black marbles than white marbles. Find the number of white marbles at first.

	Black	White
Before	5 u	4 u
Change	- 2 u	+ 0.8 u
After	3 u	4.8 u

Number of black marbles in percentage
= 100% + 25%
= 125%

125% = ¹²⁵₁₀₀ = ⁵₄

Number of black marbles that Andy removed
= 40% x 5 u
= ⁴⁰₁₀₀ x 5 u 
= 2 u

Number of white marbles that Andy increased 
= 20% x 4 u
= ²⁰₁₀₀  x 4 u
= 0.8 u

Number of black marbles in the end
= 5 u - 2 u
= 3 u

Number of white marbles in the end
= 4 u + 0.8 u
= 4.8 u

Number of fewer black marbles than white marbles in the end
= 4.8 u - 3 u 
= 1.8 u 

1.8 u = 36

1 u = 36 ÷ 1.8 = 20

Number of white marbles at first
= 4 u
= 4 x 20
= 80

Answer(s): 80