A sticker, a marble and a bead costs $6.70 altogether.
The sticker costs $1.20 less than the marble.
The bead costs $3.10 more than the marble.
How much do 3 beads cost?
Choose the cheapest item to be 1 u.
The sticker costs $1.20 less than the marble.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the marble = 1 u + 1.20
Cost of the bead = 1 u + 1.20 + 3.10 = 1 u + 4.30

Total cost of the sticker, the marble and the bead
= 1 u + 1 u + 1.20 + 1 u + 4.30
= 1 u + 1 u + 1 u + 1.20 + 4.30
= 3 u + 5.5

3 u + 5.50 = 6.70
3 u = 6.70 - 5.50
3 u = 1.20

1 u = 1.20 ÷ 3 = 0.40

Cost of 3 beads
= 3 x (1 u + 4.30)
= 3 x (0.40 + 4.30)
= 3 x 4.70
= $14.10

Answer(s): $14.10