Adam pays $16.90 for 4 stickers and 3 marbles.
A sticker costs $2.30 more than 1 marble.
What is the cost of 2 stickers and 3 marbles?
Choose the cheaper item to be 1 u.
1 sticker costs $2.30 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 2.30

Cost of 4 stickers = 4 x (1 u + 2.30) = 4 u + 9.20
Cost of 3 marbles = 3 u

Total cost of 4 stickers and 3 marbles
= 4 u + 9.20 + 3 u
= 4 u + 3 u + 9.20
= 7 u + 9.20

7 u + 9.20 = 16.90
7 u = 16.90 - 9.20
7 u = 7.70

1 u = 7.70 ÷ 7 = 1.10

Cost of 2 stickers and 3 marbles
= 2 x (1 u + 2.30) + 3 u
= 2 u + 4.60 + 3 u
= 2 u + 3 u + 4.60
= 5 u + 4.60
= 5 x 1.10 + 4.60
= 5.50 + 4.60
= $10.10

Answer(s): $10.10