Adam pays $16.30 for 4 stickers and 3 marbles.
A sticker costs $3.90 more than 1 marble.
What is the cost of 3 stickers and 2 marbles?
Choose the cheaper item to be 1 u.
1 sticker costs $3.90 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 3.90

Cost of 4 stickers = 4 x (1 u + 3.90) = 4 u + 15.60
Cost of 3 marbles = 3 u

Total cost of 4 stickers and 3 marbles
= 4 u + 15.60 + 3 u
= 4 u + 3 u + 15.60
= 7 u + 15.60

7 u + 15.60 = 16.30
7 u = 16.30 - 15.60
7 u = 0.70

1 u = 0.70 ÷ 7 = 0.10

Cost of 3 stickers and 2 marbles
= 3 x (1 u + 3.90) + 2 u
= 3 u + 11.70 + 2 u
= 3 u + 2 u + 11.70
= 5 u + 11.70
= 5 x 0.10 + 11.70
= 0.50 + 11.70
= $12.20

Answer(s): $12.20