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

Number of stickers in the third box = 1 u
Number of stickers in the second box = 1 u + 20
Number of stickers in the first box = 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 second box and third box than the first box
= (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