90 stickers are in 3 boxes.
The first box has 3 times as many stickers as the third box.
The second box is 10 less than the third box.
How many less stickers are there in the third box and the second box than in the first box?
Choose the box with the least stickers to be 1 u.

Number of stickers in the second box = 1 u
Number of stickers in the third box = 1 u + 10
Number of stickers in the first box = 3 x (1 u + 10) = 3 u + 30

Total number of stickers
= 3 u + 30 + 1 u + 10 + 1 u
= 3 u + 1 u + 1 u + 30 + 10
= 5 u + 40

5 u + 40 = 90
5 u = 90 - 40
5 u = 50

1 u = 50 ÷ 5 = 10

Number of less stickers in the third box and second box than the first box
= (3 u + 30) - (1 u + 10 + 1 u)
= (3 u + 30) - (2 u + 10)
= 3 u - 2 u  + 30 - 10
= 1 u + 20
= 1 x 10 + 20
= 10 + 20
= 30

Answer(s): 30