15 days ago, Hilda had a total of 220 green and black stickers. 25% of her stickers were black stickers. Today, her older brother gave her some more stickers. As a result, her green stickers increased by 20%. Now, 50% of her stickers are black stickers. How many black stickers did her older brother give her?

	Black stickers	Green stickers	Total  stickers
Before	1 u	3 u	220
Change	+ 2.6 u	+ 0.6 u
After	3.6 u	3.6 u
Comparing the green stickers and black stickers in the end	1	1

25% = ²⁵₁₀₀ = ¹₄

Black stickers : Green stickers = 1 : 3

Increase in the number of green stickers
= 20% x 3 u
= ²⁰₁₀₀ x 3 u
= 0.6 u

50% = ⁵⁰₁₀₀ = ¹₂

In the end
Black stickers : Green stickers = 1 : 1

Number of stickers at first
= 3 u + 1 u
= 4 u 

4 u = 220

1 u = 220 ÷ 4 = 55

Number of black stickers that Hilda received from her older brother
= 3.6 u - 1 u
= 2.6 u
= 2.6 x 55
= 143

Answer(s): 143