A marble, a sticker and a bead costs $8.70 altogether.
The marble costs $1.20 less than the sticker.
The bead costs $2.40 more than the sticker.
How much do 3 beads cost?
Choose the cheapest item to be 1 u.
The marble costs $1.20 less than the sticker.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 1.20
Cost of the bead = 1 u + 1.20 + 2.40 = 1 u + 3.60

Total cost of the marble, the sticker and the bead
= 1 u + 1 u + 1.20 + 1 u + 3.60
= 1 u + 1 u + 1 u + 1.20 + 3.60
= 3 u + 4.8

3 u + 4.80 = 8.70
3 u = 8.70 - 4.80
3 u = 3.90

1 u = 3.90 ÷ 3 = 1.30

Cost of 3 beads
= 3 x (1 u + 3.60)
= 3 x (1.30 + 3.60)
= 3 x 4.90
= $14.70

Answer(s): $14.70