Adam pays $19.80 for a marble, a bead and a sticker.
The marble costs $2.30 less than the bead.
The bead costs $1.40 less than the sticker.
What is the cost of 6 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 + 2.30
Cost of the sticker = 1 u + 2.30 + 1.40 = 1 u + 3.70

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 2.30 + 1 u + 3.70
= 3 u + 6.00

3 u + 6.00 = 19.80
3 u = 19.80 - 6.00
3 u = 13.80

1 u = 13.80 ÷ 3 = 4.60

Cost of 6 marbles less than 8 stickers 
= 8 x (1 u + 3.70) - 6 x 1 u
= 8 u + 29.60 - 6 u
= 2 u + 29.60
= 2 x 4.60 + 29.60
= 9.20 + 29.60
= $38.80

Answer(s): $38.80