Adam has 90 green stickers and 210 red stickers.
After buying the same number of stickers,
the ratio of the green stickers and the red stickers becomes 3 : 5.
How many stickers does he buy?

	Green	Red	Difference
Before	90	210	120
Change	+ ?	+ ?
After	3 u	5 u	2 u

At first
Difference in the number of stickers
= 210 - 90
= 120

In the end
Number of green stickers = 3 u
Number of red stickers = 5 u

Difference in the number of stickers in units
= 5 u - 3 u
= 2 u

The difference in the number of stickers between the green stickers and the red stickers remains unchanged.

2 u = 120

1 u = 120 ÷ 2 = 60

Number of green stickers that Adam buys
= 3 u - 90
= 3 x 60 - 90
= 180 - 90
= 90

Total number of stickers that Adam buys
= 2 x 90
= 180

Answer(s): 180