A bead, a sticker and a marble costs $9.80 altogether.
The bead costs $2.50 less than the sticker.
The marble costs $3.90 more than the sticker.
How much do 2 marbles cost?
Choose the cheapest item to be 1 u.
The bead costs $2.50 less than the sticker.
The cheapest item is the bead.

Cost of the bead = 1 u
Cost of the sticker = 1 u + 2.50
Cost of the marble = 1 u + 2.50 + 3.90 = 1 u + 6.40

Total cost of the bead, the sticker and the marble
= 1 u + 1 u + 2.50 + 1 u + 6.40
= 1 u + 1 u + 1 u + 2.50 + 6.40
= 3 u + 8.9

3 u + 8.90 = 9.80
3 u = 9.80 - 8.90
3 u = 0.90

1 u = 0.90 ÷ 3 = 0.30

Cost of 2 marbles
= 2 x (1 u + 6.40)
= 2 x (0.30 + 6.40)
= 2 x 6.70
= $13.40

Answer(s): $13.40