Adam pays $19.20 for 4 marbles and 2 stickers.
A marble costs $2.70 more than 1 sticker.
What is the cost of 2 marbles and 4 stickers?
Choose the cheaper item to be 1 u.
1 marble costs $2.70 more than 1 sticker.
The cheaper item is the sticker.

Cost of the sticker = 1 u
Cost of the marble = 1 u + 2.70

Cost of 4 marbles = 4 x (1 u + 2.70) = 4 u + 10.80
Cost of 2 stickers = 2 u

Total cost of 4 marbles and 2 stickers
= 4 u + 10.80 + 2 u
= 4 u + 2 u + 10.80
= 6 u + 10.80

6 u + 10.80 = 19.20
6 u = 19.20 - 10.80
6 u = 8.40

1 u = 8.40 ÷ 6 = 1.40

Cost of 2 marbles and 4 stickers
= 2 x (1 u + 2.70) + 4 u
= 2 u + 5.40 + 4 u
= 2 u + 4 u + 5.40
= 6 u + 5.40
= 6 x 1.40 + 5.40
= 8.40 + 5.40
= $13.80

Answer(s): $13.80