A bead, a sticker and a marble costs $9.70 altogether.
The bead costs $1.20 less than the sticker.
The marble costs $4.90 more than the sticker.
How much do 2 marbles cost?
Choose the cheapest item to be 1 u.
The bead costs $1.20 less than the sticker.
The cheapest item is the bead.

Cost of the bead = 1 u
Cost of the sticker = 1 u + 1.20
Cost of the marble = 1 u + 1.20 + 4.90 = 1 u + 6.10

Total cost of the bead, the sticker and the marble
= 1 u + 1 u + 1.20 + 1 u + 6.10
= 1 u + 1 u + 1 u + 1.20 + 6.10
= 3 u + 7.3

3 u + 7.30 = 9.70
3 u = 9.70 - 7.30
3 u = 2.40

1 u = 2.40 ÷ 3 = 0.80

Cost of 2 marbles
= 2 x (1 u + 6.10)
= 2 x (0.80 + 6.10)
= 2 x 6.90
= $13.80

Answer(s): $13.80