A sticker, a bead and a marble costs $4.20 altogether.
The sticker costs $1.40 less than the bead.
The marble costs $1.10 more than the bead.
How much do 4 marbles cost?
Choose the cheapest item to be 1 u.
The sticker costs $1.40 less than the bead.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the bead = 1 u + 1.40
Cost of the marble = 1 u + 1.40 + 1.10 = 1 u + 2.50

Total cost of the sticker, the bead and the marble
= 1 u + 1 u + 1.40 + 1 u + 2.50
= 1 u + 1 u + 1 u + 1.40 + 2.50
= 3 u + 3.9

3 u + 3.90 = 4.20
3 u = 4.20 - 3.90
3 u = 0.30

1 u = 0.30 ÷ 3 = 0.10

Cost of 4 marbles
= 4 x (1 u + 2.50)
= 4 x (0.10 + 2.50)
= 4 x 2.60
= $10.40

Answer(s): $10.40