Adam pays $18.60 for 4 marbles and 2 stickers.
A marble costs $2.70 more than 1 sticker.
What is the cost of 3 marbles and 2 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 = 18.60
6 u = 18.60 - 10.80
6 u = 7.80

1 u = 7.80 ÷ 6 = 1.30

Cost of 3 marbles and 2 stickers
= 3 x (1 u + 2.70) + 2 u
= 3 u + 8.10 + 2 u
= 3 u + 2 u + 8.10
= 5 u + 8.10
= 5 x 1.30 + 8.10
= 6.50 + 8.10
= $14.60

Answer(s): $14.60