There are 25% more grey coins than blue coins in a bag. After Charlie removed 20% of the grey coins and increased the number of blue coins by 30%, there are now 120 fewer grey coins than blue coins. Find the number of blue coins at first.

	Grey	Blue
Before	5 u	4 u
Change	- 1 u	+ 1.2 u
After	4 u	5.2 u

Number of grey coins in percentage
= 100% + 25%
= 125%

125% = ¹²⁵₁₀₀ = ⁵₄

Number of grey coins that Charlie removed
= 20% x 5 u
= ²⁰₁₀₀ x 5 u 
= 1 u

Number of blue coins that Charlie increased 
= 30% x 4 u
= ³⁰₁₀₀  x 4 u
= 1.2 u

Number of grey coins in the end
= 5 u - 1 u
= 4 u

Number of blue coins in the end
= 4 u + 1.2 u
= 5.2 u

Number of fewer grey coins than blue coins in the end
= 5.2 u - 4 u 
= 1.2 u 

1.2 u = 120

1 u = 120 ÷ 1.2 = 100

Number of blue coins at first
= 4 u
= 4 x 100
= 400

Answer(s): 400