4000 stickers are in 3 bags.
The 1st bag has 400 stickers more than the total number of stickers in the 2nd and 3rd bags.
The 3rd bag has 4 times as many stickers as the 2nd bag.
How many stickers does the 1st 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 3rd bag = 4 u

Number of stickers in the 2nd bag and 3rd bag
= 1 u + 4 u
= 5 u

Number of stickers in the 1st bag = 5 u + 400

Total number of stickers
= 1 u + 4 u + 5 u + 400
= 10 u + 400

10 u + 400 = 4000
10 u = 4000 - 400
10 u = 3600

1 u = 3600 ÷ 10 = 360

Number of stickers in the 1st bag
= 5 u + 400
= 5 x 360 + 400
= 1800 + 400
= 2200

Answer(s): 2200