A sticker, a bead and a marble costs $6.10 altogether.
The sticker costs $1.30 less than the bead.
The marble costs $2.60 more than the bead.
How much do 2 marbles cost?
Choose the cheapest item to be 1 u.
The sticker costs $1.30 less than the bead.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the bead = 1 u + 1.30
Cost of the marble = 1 u + 1.30 + 2.60 = 1 u + 3.90

Total cost of the sticker, the bead and the marble
= 1 u + 1 u + 1.30 + 1 u + 3.90
= 1 u + 1 u + 1 u + 1.30 + 3.90
= 3 u + 5.2

3 u + 5.20 = 6.10
3 u = 6.10 - 5.20
3 u = 0.90

1 u = 0.90 ÷ 3 = 0.30

Cost of 2 marbles
= 2 x (1 u + 3.90)
= 2 x (0.30 + 3.90)
= 2 x 4.20
= $8.40

Answer(s): $8.40