A sticker, a bead and a marble costs $9.80 altogether.
The sticker costs $3.80 less than the bead.
The marble costs $1.90 more than the bead.
How much do 2 marbles cost?
Choose the cheapest item to be 1 u.
The sticker costs $3.80 less than the bead.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the bead = 1 u + 3.80
Cost of the marble = 1 u + 3.80 + 1.90 = 1 u + 5.70

Total cost of the sticker, the bead and the marble
= 1 u + 1 u + 3.80 + 1 u + 5.70
= 1 u + 1 u + 1 u + 3.80 + 5.70
= 3 u + 9.5

3 u + 9.50 = 9.80
3 u = 9.80 - 9.50
3 u = 0.30

1 u = 0.30 ÷ 3 = 0.10

Cost of 2 marbles
= 2 x (1 u + 5.70)
= 2 x (0.10 + 5.70)
= 2 x 5.80
= $11.60

Answer(s): $11.60