Adam pays $18.60 for 3 stickers and 4 marbles.
A sticker costs $1.30 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 $1.30 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 1.30

Cost of 3 stickers = 3 x (1 u + 1.30) = 3 u + 3.90
Cost of 4 marbles = 4 u

Total cost of 3 stickers and 4 marbles
= 3 u + 3.90 + 4 u
= 3 u + 4 u + 3.90
= 7 u + 3.90

7 u + 3.90 = 18.60
7 u = 18.60 - 3.90
7 u = 14.70

1 u = 14.70 ÷ 7 = 2.10

Cost of 4 stickers and 3 marbles
= 4 x (1 u + 1.30) + 3 u
= 4 u + 5.20 + 3 u
= 4 u + 3 u + 5.20
= 7 u + 5.20
= 7 x 2.10 + 5.20
= 14.70 + 5.20
= $19.90

Answer(s): $19.90