Adam pays $3.80 for 2 stickers and 4 marbles.
A sticker costs $1.60 more than 1 marble.
What is the cost of 3 stickers and 4 marbles?
Choose the cheaper item to be 1 u.
1 sticker costs $1.60 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 1.60

Cost of 2 stickers = 2 x (1 u + 1.60) = 2 u + 3.20
Cost of 4 marbles = 4 u

Total cost of 2 stickers and 4 marbles
= 2 u + 3.20 + 4 u
= 2 u + 4 u + 3.20
= 6 u + 3.20

6 u + 3.20 = 3.80
6 u = 3.80 - 3.20
6 u = 0.60

1 u = 0.60 ÷ 6 = 0.10

Cost of 3 stickers and 4 marbles
= 3 x (1 u + 1.60) + 4 u
= 3 u + 4.80 + 4 u
= 3 u + 4 u + 4.80
= 7 u + 4.80
= 7 x 0.10 + 4.80
= 0.70 + 4.80
= $5.50

Answer(s): $5.50