A sticker, a bead and a marble costs $7.30 altogether.
The sticker costs $2.20 less than the bead.
The marble costs $1.70 more than the bead.
How much do 3 marbles cost?
Choose the cheapest item to be 1 u.
The sticker costs $2.20 less than the bead.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the bead = 1 u + 2.20
Cost of the marble = 1 u + 2.20 + 1.70 = 1 u + 3.90

Total cost of the sticker, the bead and the marble
= 1 u + 1 u + 2.20 + 1 u + 3.90
= 1 u + 1 u + 1 u + 2.20 + 3.90
= 3 u + 6.1

3 u + 6.10 = 7.30
3 u = 7.30 - 6.10
3 u = 1.20

1 u = 1.20 ÷ 3 = 0.40

Cost of 3 marbles
= 3 x (1 u + 3.90)
= 3 x (0.40 + 3.90)
= 3 x 4.30
= $12.90

Answer(s): $12.90