90 stickers are in 3 bags.
The first bag has 3 times as many stickers as the third bag.
The second bag is 20 less than the third bag.
How many less stickers are there in the third bag and the second bag than in the first bag?
Choose the bag with the least stickers to be 1 u.

Number of stickers in the second bag = 1 u
Number of stickers in the third bag = 1 u + 20
Number of stickers in the first bag = 3 x (1 u + 20) = 3 u + 60

Total number of stickers
= 3 u + 60 + 1 u + 20 + 1 u
= 3 u + 1 u + 1 u + 60 + 20
= 5 u + 80

5 u + 80 = 90
5 u = 90 - 80
5 u = 10

1 u = 10 ÷ 5 = 2

Number of less stickers in the third bag and second bag than the first bag
= (3 u + 60) - (1 u + 20 + 1 u)
= (3 u + 60) - (2 u + 20)
= 3 u - 2 u  + 60 - 20
= 1 u + 40
= 1 x 2 + 40
= 2 + 40
= 42

Answer(s): 42