Adam pays $5.10 for 2 stickers and 3 marbles.
A sticker costs $1.80 more than 1 marble.
What is the cost of 4 stickers and 3 marbles?
Choose the cheaper item to be 1 u.
1 sticker costs $1.80 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 1.80

Cost of 2 stickers = 2 x (1 u + 1.80) = 2 u + 3.60
Cost of 3 marbles = 3 u

Total cost of 2 stickers and 3 marbles
= 2 u + 3.60 + 3 u
= 2 u + 3 u + 3.60
= 5 u + 3.60

5 u + 3.60 = 5.10
5 u = 5.10 - 3.60
5 u = 1.50

1 u = 1.50 ÷ 5 = 0.30

Cost of 4 stickers and 3 marbles
= 4 x (1 u + 1.80) + 3 u
= 4 u + 7.20 + 3 u
= 4 u + 3 u + 7.20
= 7 u + 7.20
= 7 x 0.30 + 7.20
= 2.10 + 7.20
= $9.30

Answer(s): $9.30