A marble, a bead and a sticker costs $9.10 altogether.
The marble costs $2.30 less than the bead.
The sticker costs $4.20 more than the bead.
How much do 3 stickers cost?
Choose the cheapest item to be 1 u.
The marble costs $2.30 less than the bead.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the bead = 1 u + 2.30
Cost of the sticker = 1 u + 2.30 + 4.20 = 1 u + 6.50

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 2.30 + 1 u + 6.50
= 1 u + 1 u + 1 u + 2.30 + 6.50
= 3 u + 8.8

3 u + 8.80 = 9.10
3 u = 9.10 - 8.80
3 u = 0.30

1 u = 0.30 ÷ 3 = 0.10

Cost of 3 stickers
= 3 x (1 u + 6.50)
= 3 x (0.10 + 6.50)
= 3 x 6.60
= $19.80

Answer(s): $19.80