Adam pays $28.80 for a marble, a bead and a sticker.
The marble costs $2.40 less than the bead.
The bead costs $1.80 less than the sticker.
What is the cost of 9 stickers more than 4 marbles?
Choose the cheapest item to be 1 u.

Cost of the marble = 1 u
Cost of the bead = 1 u + 2.40
Cost of the sticker = 1 u + 2.40 + 1.80 = 1 u + 4.20

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 2.40 + 1 u + 4.20
= 3 u + 6.60

3 u + 6.60 = 28.80
3 u = 28.80 - 6.60
3 u = 22.20

1 u = 22.20 ÷ 3 = 7.40

Cost of 9 stickers more than 4 marbles
= 9 x (1 u + 4.20) - 4 x 1 u
= 9 u + 37.80 - 4 u
= 5 u + 37.80
= 5 x 7.40 + 37.80
= 37.00 + 37.80
= $74.80

Answer(s): $74.80