Adam pays $20.30 for a marble, a bead and a sticker.
The marble costs $3.50 less than the bead.
The bead costs $1.90 less than the sticker.
What is the cost of 3 marbles less than 6 stickers?
Choose the cheapest item to be 1 u.

Cost of the marble = 1 u
Cost of the bead = 1 u + 3.50
Cost of the sticker = 1 u + 3.50 + 1.90 = 1 u + 5.40

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 3.50 + 1 u + 5.40
= 3 u + 8.90

3 u + 8.90 = 20.30
3 u = 20.30 - 8.90
3 u = 11.40

1 u = 11.40 ÷ 3 = 3.80

Cost of 3 marbles less than 6 stickers 
= 6 x (1 u + 5.40) - 3 x 1 u
= 6 u + 32.40 - 3 u
= 3 u + 32.40
= 3 x 3.80 + 32.40
= 11.40 + 32.40
= $43.80

Answer(s): $43.80