There are 25% more white buttons than black buttons in a bag. After Peter removed 20% of the white buttons and increased the number of black buttons by 10%, there are now 12 fewer white buttons than black buttons. Find the number of black buttons at first.

	White	Black
Before	5 u	4 u
Change	- 1 u	+ 0.4 u
After	4 u	4.4 u

Number of white buttons in percentage
= 100% + 25%
= 125%

125% = ¹²⁵₁₀₀ = ⁵₄

Number of white buttons that Peter removed
= 20% x 5 u
= ²⁰₁₀₀ x 5 u 
= 1 u

Number of black buttons that Peter increased 
= 10% x 4 u
= ¹⁰₁₀₀  x 4 u
= 0.4 u

Number of white buttons in the end
= 5 u - 1 u
= 4 u

Number of black buttons in the end
= 4 u + 0.4 u
= 4.4 u

Number of fewer white buttons than black buttons in the end
= 4.4 u - 4 u 
= 0.4 u 

0.4 u = 12

1 u = 12 ÷ 0.4 = 30

Number of black buttons at first
= 4 u
= 4 x 30
= 120

Answer(s): 120