Adam pays $18.10 for 4 stickers and 3 marbles.
A sticker costs $1.20 more than 1 marble.
What is the cost of 2 stickers and 3 marbles?
Choose the cheaper item to be 1 u.
1 sticker costs $1.20 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 1.20

Cost of 4 stickers = 4 x (1 u + 1.20) = 4 u + 4.80
Cost of 3 marbles = 3 u

Total cost of 4 stickers and 3 marbles
= 4 u + 4.80 + 3 u
= 4 u + 3 u + 4.80
= 7 u + 4.80

7 u + 4.80 = 18.10
7 u = 18.10 - 4.80
7 u = 13.30

1 u = 13.30 ÷ 7 = 1.90

Cost of 2 stickers and 3 marbles
= 2 x (1 u + 1.20) + 3 u
= 2 u + 2.40 + 3 u
= 2 u + 3 u + 2.40
= 5 u + 2.40
= 5 x 1.90 + 2.40
= 9.50 + 2.40
= $11.90

Answer(s): $11.90