Adam pays $21.60 for a marble, a bead and a sticker.
The marble costs $3.40 less than the bead.
The bead costs $1.30 less than the sticker.
What is the cost of 7 marbles less than 8 stickers?
Choose the cheapest item to be 1 u.

Cost of the marble = 1 u
Cost of the bead = 1 u + 3.40
Cost of the sticker = 1 u + 3.40 + 1.30 = 1 u + 4.70

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 3.40 + 1 u + 4.70
= 3 u + 8.10

3 u + 8.10 = 21.60
3 u = 21.60 - 8.10
3 u = 13.50

1 u = 13.50 ÷ 3 = 4.50

Cost of 7 marbles less than 8 stickers 
= 8 x (1 u + 4.70) - 7 x 1 u
= 8 u + 37.60 - 7 u
= 1 u + 37.60
= 1 x 4.50 + 37.60
= 4.50 + 37.60
= $42.10

Answer(s): $42.10