Adam pays $15.90 for 4 marbles and 3 stickers.
A marble costs $1.70 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 $1.70 more than 1 sticker.
The cheaper item is the sticker.

Cost of the sticker = 1 u
Cost of the marble = 1 u + 1.70

Cost of 4 marbles = 4 x (1 u + 1.70) = 4 u + 6.80
Cost of 3 stickers = 3 u

Total cost of 4 marbles and 3 stickers
= 4 u + 6.80 + 3 u
= 4 u + 3 u + 6.80
= 7 u + 6.80

7 u + 6.80 = 15.90
7 u = 15.90 - 6.80
7 u = 9.10

1 u = 9.10 ÷ 7 = 1.30

Cost of 2 marbles and 4 stickers
= 2 x (1 u + 1.70) + 4 u
= 2 u + 3.40 + 4 u
= 2 u + 4 u + 3.40
= 6 u + 3.40
= 6 x 1.30 + 3.40
= 7.80 + 3.40
= $11.20

Answer(s): $11.20