Adam pays $18.90 for 4 stickers and 3 marbles.
A sticker costs $2.10 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 $2.10 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 2.10

Cost of 4 stickers = 4 x (1 u + 2.10) = 4 u + 8.40
Cost of 3 marbles = 3 u

Total cost of 4 stickers and 3 marbles
= 4 u + 8.40 + 3 u
= 4 u + 3 u + 8.40
= 7 u + 8.40

7 u + 8.40 = 18.90
7 u = 18.90 - 8.40
7 u = 10.50

1 u = 10.50 ÷ 7 = 1.50

Cost of 3 stickers and 4 marbles
= 3 x (1 u + 2.10) + 4 u
= 3 u + 6.30 + 4 u
= 3 u + 4 u + 6.30
= 7 u + 6.30
= 7 x 1.50 + 6.30
= 10.50 + 6.30
= $16.80

Answer(s): $16.80