A sticker, a bead and a marble costs $7.60 altogether.
The sticker costs $2.10 less than the bead.
The marble costs $3.10 more than the bead.
How much do 3 marbles cost?
Choose the cheapest item to be 1 u.
The sticker costs $2.10 less than the bead.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the bead = 1 u + 2.10
Cost of the marble = 1 u + 2.10 + 3.10 = 1 u + 5.20

Total cost of the sticker, the bead and the marble
= 1 u + 1 u + 2.10 + 1 u + 5.20
= 1 u + 1 u + 1 u + 2.10 + 5.20
= 3 u + 7.3

3 u + 7.30 = 7.60
3 u = 7.60 - 7.30
3 u = 0.30

1 u = 0.30 ÷ 3 = 0.10

Cost of 3 marbles
= 3 x (1 u + 5.20)
= 3 x (0.10 + 5.20)
= 3 x 5.30
= $15.90

Answer(s): $15.90