Adam pays $19.80 for a marble, a bead and a sticker.
The marble costs $3.20 less than the bead.
The bead costs $1.70 less than the sticker.
What is the cost of 6 marbles less than 7 stickers?
Choose the cheapest item to be 1 u.

Cost of the marble = 1 u
Cost of the bead = 1 u + 3.20
Cost of the sticker = 1 u + 3.20 + 1.70 = 1 u + 4.90

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 3.20 + 1 u + 4.90
= 3 u + 8.10

3 u + 8.10 = 19.80
3 u = 19.80 - 8.10
3 u = 11.70

1 u = 11.70 ÷ 3 = 3.90

Cost of 6 marbles less than 7 stickers 
= 7 x (1 u + 4.90) - 6 x 1 u
= 7 u + 34.30 - 6 u
= 1 u + 34.30
= 1 x 3.90 + 34.30
= 3.90 + 34.30
= $38.20

Answer(s): $38.20