Adam pays $21.90 for a marble, a bead and a sticker.
The marble costs $2.10 less than the bead.
The bead costs $1.80 less than the sticker.
What is the cost of 4 stickers more than 2 marbles?
Choose the cheapest item to be 1 u.

Cost of the marble = 1 u
Cost of the bead = 1 u + 2.10
Cost of the sticker = 1 u + 2.10 + 1.80 = 1 u + 3.90

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 2.10 + 1 u + 3.90
= 3 u + 6.00

3 u + 6.00 = 21.90
3 u = 21.90 - 6.00
3 u = 15.90

1 u = 15.90 ÷ 3 = 5.30

Cost of 4 stickers more than 2 marbles
= 4 x (1 u + 3.90) - 2 x 1 u
= 4 u + 15.60 - 2 u
= 2 u + 15.60
= 2 x 5.30 + 15.60
= 10.60 + 15.60
= $26.20

Answer(s): $26.20