A marble, a bead and a sticker costs $9.90 altogether.
The marble costs $2.80 less than the bead.
The sticker costs $3.70 more than the bead.
How much do 2 stickers cost?
Choose the cheapest item to be 1 u.
The marble costs $2.80 less than the bead.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the bead = 1 u + 2.80
Cost of the sticker = 1 u + 2.80 + 3.70 = 1 u + 6.50

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 2.80 + 1 u + 6.50
= 1 u + 1 u + 1 u + 2.80 + 6.50
= 3 u + 9.3

3 u + 9.30 = 9.90
3 u = 9.90 - 9.30
3 u = 0.60

1 u = 0.60 ÷ 3 = 0.20

Cost of 2 stickers
= 2 x (1 u + 6.50)
= 2 x (0.20 + 6.50)
= 2 x 6.70
= $13.40

Answer(s): $13.40