A sticker, a bead and a marble costs $7.90 altogether.
The sticker costs $1.90 less than the bead.
The marble costs $2.90 more than the bead.
How much do 2 marbles cost?
Choose the cheapest item to be 1 u.
The sticker costs $1.90 less than the bead.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the bead = 1 u + 1.90
Cost of the marble = 1 u + 1.90 + 2.90 = 1 u + 4.80

Total cost of the sticker, the bead and the marble
= 1 u + 1 u + 1.90 + 1 u + 4.80
= 1 u + 1 u + 1 u + 1.90 + 4.80
= 3 u + 6.7

3 u + 6.70 = 7.90
3 u = 7.90 - 6.70
3 u = 1.20

1 u = 1.20 ÷ 3 = 0.40

Cost of 2 marbles
= 2 x (1 u + 4.80)
= 2 x (0.40 + 4.80)
= 2 x 5.20
= $10.40

Answer(s): $10.40