A bead, a sticker and a marble costs $9.10 altogether.
The bead costs $2.10 less than the sticker.
The marble costs $2.20 more than the sticker.
How much do 3 marbles cost?
Choose the cheapest item to be 1 u.
The bead costs $2.10 less than the sticker.
The cheapest item is the bead.

Cost of the bead = 1 u
Cost of the sticker = 1 u + 2.10
Cost of the marble = 1 u + 2.10 + 2.20 = 1 u + 4.30

Total cost of the bead, the sticker and the marble
= 1 u + 1 u + 2.10 + 1 u + 4.30
= 1 u + 1 u + 1 u + 2.10 + 4.30
= 3 u + 6.4

3 u + 6.40 = 9.10
3 u = 9.10 - 6.40
3 u = 2.70

1 u = 2.70 ÷ 3 = 0.90

Cost of 3 marbles
= 3 x (1 u + 4.30)
= 3 x (0.90 + 4.30)
= 3 x 5.20
= $15.60

Answer(s): $15.60