There are 25% more black pens than red pens in a bag. After Ethan removed 20% of the black pens and increased the number of red pens by 20%, there are now 64 fewer black pens than red pens. Find the number of red pens at first.

	Black	Red
Before	5 u	4 u
Change	- 1 u	+ 0.8 u
After	4 u	4.8 u

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

125% = ¹²⁵₁₀₀ = ⁵₄

Number of black pens that Ethan removed
= 20% x 5 u
= ²⁰₁₀₀ x 5 u 
= 1 u

Number of red pens that Ethan increased 
= 20% x 4 u
= ²⁰₁₀₀  x 4 u
= 0.8 u

Number of black pens in the end
= 5 u - 1 u
= 4 u

Number of red pens in the end
= 4 u + 0.8 u
= 4.8 u

Number of fewer black pens than red pens in the end
= 4.8 u - 4 u 
= 0.8 u 

0.8 u = 64

1 u = 64 ÷ 0.8 = 80

Number of red pens at first
= 4 u
= 4 x 80
= 320

Answer(s): 320