A marble, a bead and a sticker costs $8.50 altogether.
The marble costs $3.20 less than the bead.
The sticker costs $1.20 more than the bead.
How much do 4 stickers cost?
Choose the cheapest item to be 1 u.
The marble costs $3.20 less than the bead.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the bead = 1 u + 3.20
Cost of the sticker = 1 u + 3.20 + 1.20 = 1 u + 4.40

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 3.20 + 1 u + 4.40
= 1 u + 1 u + 1 u + 3.20 + 4.40
= 3 u + 7.6

3 u + 7.60 = 8.50
3 u = 8.50 - 7.60
3 u = 0.90

1 u = 0.90 ÷ 3 = 0.30

Cost of 4 stickers
= 4 x (1 u + 4.40)
= 4 x (0.30 + 4.40)
= 4 x 4.70
= $18.80

Answer(s): $18.80