Adam pays $17.60 for a marble, a bead and a sticker.
The marble costs $2.50 less than the bead.
The bead costs $1.80 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 + 2.50
Cost of the sticker = 1 u + 2.50 + 1.80 = 1 u + 4.30

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 2.50 + 1 u + 4.30
= 3 u + 6.80

3 u + 6.80 = 17.60
3 u = 17.60 - 6.80
3 u = 10.80

1 u = 10.80 ÷ 3 = 3.60

Cost of 3 marbles less than 4 stickers 
= 4 x (1 u + 4.30) - 3 x 1 u
= 4 u + 17.20 - 3 u
= 1 u + 17.20
= 1 x 3.60 + 17.20
= 3.60 + 17.20
= $20.80

Answer(s): $20.80