Adam pays $13.70 for 4 marbles and 3 stickers.
A marble costs $2.20 more than 1 sticker.
What is the cost of 2 marbles and 3 stickers?
Choose the cheaper item to be 1 u.
1 marble costs $2.20 more than 1 sticker.
The cheaper item is the sticker.

Cost of the sticker = 1 u
Cost of the marble = 1 u + 2.20

Cost of 4 marbles = 4 x (1 u + 2.20) = 4 u + 8.80
Cost of 3 stickers = 3 u

Total cost of 4 marbles and 3 stickers
= 4 u + 8.80 + 3 u
= 4 u + 3 u + 8.80
= 7 u + 8.80

7 u + 8.80 = 13.70
7 u = 13.70 - 8.80
7 u = 4.90

1 u = 4.90 ÷ 7 = 0.70

Cost of 2 marbles and 3 stickers
= 2 x (1 u + 2.20) + 3 u
= 2 u + 4.40 + 3 u
= 2 u + 3 u + 4.40
= 5 u + 4.40
= 5 x 0.70 + 4.40
= 3.50 + 4.40
= $7.90

Answer(s): $7.90