Adam pays $29.90 for a marble, a bead and a sticker.
The marble costs $3.80 less than the bead.
The bead costs $1.90 less than the sticker.
What is the cost of 4 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 + 3.80
Cost of the sticker = 1 u + 3.80 + 1.90 = 1 u + 5.70

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 3.80 + 1 u + 5.70
= 3 u + 9.50

3 u + 9.50 = 29.90
3 u = 29.90 - 9.50
3 u = 20.40

1 u = 20.40 ÷ 3 = 6.80

Cost of 4 marbles less than 8 stickers 
= 8 x (1 u + 5.70) - 4 x 1 u
= 8 u + 45.60 - 4 u
= 4 u + 45.60
= 4 x 6.80 + 45.60
= 27.20 + 45.60
= $72.80

Answer(s): $72.80