A sticker, a bead and a marble costs $8.40 altogether.
The sticker costs $1.10 less than the bead.
The marble costs $2.30 more than the bead.
How much do 2 marbles cost?
Choose the cheapest item to be 1 u.
The sticker costs $1.10 less than the bead.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the bead = 1 u + 1.10
Cost of the marble = 1 u + 1.10 + 2.30 = 1 u + 3.40

Total cost of the sticker, the bead and the marble
= 1 u + 1 u + 1.10 + 1 u + 3.40
= 1 u + 1 u + 1 u + 1.10 + 3.40
= 3 u + 4.5

3 u + 4.50 = 8.40
3 u = 8.40 - 4.50
3 u = 3.90

1 u = 3.90 ÷ 3 = 1.30

Cost of 2 marbles
= 2 x (1 u + 3.40)
= 2 x (1.30 + 3.40)
= 2 x 4.70
= $9.40

Answer(s): $9.40