A marble, a bead and a sticker costs $6.50 altogether.
The marble costs $1.40 less than the bead.
The sticker costs $3.10 more than the bead.
How much do 3 stickers cost?
Choose the cheapest item to be 1 u.
The marble costs $1.40 less than the bead.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the bead = 1 u + 1.40
Cost of the sticker = 1 u + 1.40 + 3.10 = 1 u + 4.50

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 1.40 + 1 u + 4.50
= 1 u + 1 u + 1 u + 1.40 + 4.50
= 3 u + 5.9

3 u + 5.90 = 6.50
3 u = 6.50 - 5.90
3 u = 0.60

1 u = 0.60 ÷ 3 = 0.20

Cost of 3 stickers
= 3 x (1 u + 4.50)
= 3 x (0.20 + 4.50)
= 3 x 4.70
= $14.10

Answer(s): $14.10