There are 25% more yellow buttons than grey buttons in a bag. After David removed 20% of the yellow buttons and increased the number of grey buttons by 10%, there are now 8 fewer yellow buttons than grey buttons. Find the number of grey buttons at first.

	Yellow	Grey
Before	5 u	4 u
Change	- 1 u	+ 0.4 u
After	4 u	4.4 u

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

125% = ¹²⁵₁₀₀ = ⁵₄

Number of yellow buttons that David removed
= 20% x 5 u
= ²⁰₁₀₀ x 5 u 
= 1 u

Number of grey buttons that David increased 
= 10% x 4 u
= ¹⁰₁₀₀  x 4 u
= 0.4 u

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

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

Number of fewer yellow buttons than grey buttons in the end
= 4.4 u - 4 u 
= 0.4 u 

0.4 u = 8

1 u = 8 ÷ 0.4 = 20

Number of grey buttons at first
= 4 u
= 4 x 20
= 80

Answer(s): 80