Adam pays $16.70 for 2 stickers and 3 marbles.
A sticker costs $3.10 more than 1 marble.
What is the cost of 4 stickers and 3 marbles?
Choose the cheaper item to be 1 u.
1 sticker costs $3.10 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 3.10

Cost of 2 stickers = 2 x (1 u + 3.10) = 2 u + 6.20
Cost of 3 marbles = 3 u

Total cost of 2 stickers and 3 marbles
= 2 u + 6.20 + 3 u
= 2 u + 3 u + 6.20
= 5 u + 6.20

5 u + 6.20 = 16.70
5 u = 16.70 - 6.20
5 u = 10.50

1 u = 10.50 ÷ 5 = 2.10

Cost of 4 stickers and 3 marbles
= 4 x (1 u + 3.10) + 3 u
= 4 u + 12.40 + 3 u
= 4 u + 3 u + 12.40
= 7 u + 12.40
= 7 x 2.10 + 12.40
= 14.70 + 12.40
= $27.10

Answer(s): $27.10