3100 stickers are in 3 bags.
The 2nd bag has 700 stickers more than the total number of stickers in the 3rd and 1st bags.
The 1st bag has 3 times as many stickers as the 3rd bag.
How many stickers does the 2nd bag contain?
Choose the bag with the least stickers to be 1 u.

Number of stickers in the 3rd bag = 1 u
Number of stickers in the 1st bag = 3 u

Number of stickers in the 3rd bag and 1st bag
= 1 u + 3 u
= 4 u

Number of stickers in the 2nd bag = 4 u + 700

Total number of stickers
= 1 u + 3 u + 4 u + 700
= 8 u + 700

8 u + 700 = 3100
8 u = 3100 - 700
8 u = 2400

1 u = 2400 ÷ 8 = 300

Number of stickers in the 2nd bag
= 4 u + 700
= 4 x 300 + 700
= 1200 + 700
= 1900

Answer(s): 1900