A bead, a sticker and a marble costs $8.50 altogether.
The bead costs $1.20 less than the sticker.
The marble costs $2.50 more than the sticker.
How much do 4 marbles cost?
Choose the cheapest item to be 1 u.
The bead costs $1.20 less than the sticker.
The cheapest item is the bead.

Cost of the bead = 1 u
Cost of the sticker = 1 u + 1.20
Cost of the marble = 1 u + 1.20 + 2.50 = 1 u + 3.70

Total cost of the bead, the sticker and the marble
= 1 u + 1 u + 1.20 + 1 u + 3.70
= 1 u + 1 u + 1 u + 1.20 + 3.70
= 3 u + 4.9

3 u + 4.90 = 8.50
3 u = 8.50 - 4.90
3 u = 3.60

1 u = 3.60 ÷ 3 = 1.20

Cost of 4 marbles
= 4 x (1 u + 3.70)
= 4 x (1.20 + 3.70)
= 4 x 4.90
= $19.60

Answer(s): $19.60