There are 25% more pink pens than yellow pens in a bag. After Ivan removed 40% of the pink pens and increased the number of yellow pens by 20%, there are now 180 fewer pink pens than yellow pens. Find the number of yellow pens at first.

	Pink	Yellow
Before	5 u	4 u
Change	- 2 u	+ 0.8 u
After	3 u	4.8 u

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

125% = ¹²⁵₁₀₀ = ⁵₄

Number of pink pens that Ivan removed
= 40% x 5 u
= ⁴⁰₁₀₀ x 5 u 
= 2 u

Number of yellow pens that Ivan increased 
= 20% x 4 u
= ²⁰₁₀₀  x 4 u
= 0.8 u

Number of pink pens in the end
= 5 u - 2 u
= 3 u

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

Number of fewer pink pens than yellow pens in the end
= 4.8 u - 3 u 
= 1.8 u 

1.8 u = 180

1 u = 180 ÷ 1.8 = 100

Number of yellow pens at first
= 4 u
= 4 x 100
= 400

Answer(s): 400