Adam pays $19.20 for 3 stickers and 4 marbles.
A sticker costs $2.20 more than 1 marble.
What is the cost of 2 stickers and 4 marbles?
Choose the cheaper item to be 1 u.
1 sticker costs $2.20 more than 1 marble.
The cheaper item is the marble.

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

Cost of 3 stickers = 3 x (1 u + 2.20) = 3 u + 6.60
Cost of 4 marbles = 4 u

Total cost of 3 stickers and 4 marbles
= 3 u + 6.60 + 4 u
= 3 u + 4 u + 6.60
= 7 u + 6.60

7 u + 6.60 = 19.20
7 u = 19.20 - 6.60
7 u = 12.60

1 u = 12.60 ÷ 7 = 1.80

Cost of 2 stickers and 4 marbles
= 2 x (1 u + 2.20) + 4 u
= 2 u + 4.40 + 4 u
= 2 u + 4 u + 4.40
= 6 u + 4.40
= 6 x 1.80 + 4.40
= 10.80 + 4.40
= $15.20

Answer(s): $15.20