Adam pays $17.90 for a marble, a bead and a sticker.
The marble costs $3.10 less than the bead.
The bead costs $1.50 less than the sticker.
What is the cost of 3 marbles less than 4 stickers?
Choose the cheapest item to be 1 u.

Cost of the marble = 1 u
Cost of the bead = 1 u + 3.10
Cost of the sticker = 1 u + 3.10 + 1.50 = 1 u + 4.60

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 3.10 + 1 u + 4.60
= 3 u + 7.70

3 u + 7.70 = 17.90
3 u = 17.90 - 7.70
3 u = 10.20

1 u = 10.20 ÷ 3 = 3.40

Cost of 3 marbles less than 4 stickers 
= 4 x (1 u + 4.60) - 3 x 1 u
= 4 u + 18.40 - 3 u
= 1 u + 18.40
= 1 x 3.40 + 18.40
= 3.40 + 18.40
= $21.80

Answer(s): $21.80