A sticker, a bead and a marble costs $8.50 altogether.
The sticker costs $2.90 less than the bead.
The marble costs $1.50 more than the bead.
How much do 4 marbles cost?
Choose the cheapest item to be 1 u.
The sticker costs $2.90 less than the bead.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the bead = 1 u + 2.90
Cost of the marble = 1 u + 2.90 + 1.50 = 1 u + 4.40

Total cost of the sticker, the bead and the marble
= 1 u + 1 u + 2.90 + 1 u + 4.40
= 1 u + 1 u + 1 u + 2.90 + 4.40
= 3 u + 7.3

3 u + 7.30 = 8.50
3 u = 8.50 - 7.30
3 u = 1.20

1 u = 1.20 ÷ 3 = 0.40

Cost of 4 marbles
= 4 x (1 u + 4.40)
= 4 x (0.40 + 4.40)
= 4 x 4.80
= $19.20

Answer(s): $19.20