Adam pays $18.90 for a marble, a bead and a sticker.
The marble costs $2.50 less than the bead.
The bead costs $1.30 less than the sticker.
What is the cost of 8 marbles less than 9 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 + 1.30 = 1 u + 3.80

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 2.50 + 1 u + 3.80
= 3 u + 6.30

3 u + 6.30 = 18.90
3 u = 18.90 - 6.30
3 u = 12.60

1 u = 12.60 ÷ 3 = 4.20

Cost of 8 marbles less than 9 stickers 
= 9 x (1 u + 3.80) - 8 x 1 u
= 9 u + 34.20 - 8 u
= 1 u + 34.20
= 1 x 4.20 + 34.20
= 4.20 + 34.20
= $38.40

Answer(s): $38.40