3500 stickers are in 3 bags.
The 3rd bag has 900 stickers more than the total number of stickers in the 2nd and 1st bags.
The 1st bag has 4 times as many stickers as the 2nd bag.
How many stickers does the 3rd bag contain?
Choose the bag with the least stickers to be 1 u.

Number of stickers in the 2nd bag = 1 u
Number of stickers in the 1st bag = 4 u

Number of stickers in the 2nd bag and 1st bag
= 1 u + 4 u
= 5 u

Number of stickers in the 3rd bag = 5 u + 900

Total number of stickers
= 1 u + 4 u + 5 u + 900
= 10 u + 900

10 u + 900 = 3500
10 u = 3500 - 900
10 u = 2600

1 u = 2600 ÷ 10 = 260

Number of stickers in the 3rd bag
= 5 u + 900
= 5 x 260 + 900
= 1300 + 900
= 2200

Answer(s): 2200