If Adam buys 3 stickers and 4 marbles, it costs $38.70.
If Adam buys 4 stickers and 3 marbles, he will need another $5.20.

1. Find the cost of 1 sticker and 1 marble.
2. Find the cost of 1 sticker.
3. Find the cost of 1 marble.

(a)
3 stickers + 4 marbles = 38.70 --- (1)
4 stickers + 3 marbles = 38.70 + 5.20 = 43.90 --- (2)

(1) + (2)
7 stickers + 7 marbles = 82.60 --- (3)

(3) _{÷ 7}
1 sticker + 1 marble = 82.60 ÷ 7 = 11.80 --- (4)

Cost of 1 sticker and 1 marble = $11.80

(b)
4 stickers + 3 marbles = 43.90 --- (2)

(4) _{x 3}
3 stickers + 3 marbles = 3 x 11.80 = 35.40 --- (5)

(2) - (5)
4 stickers + 3 marbles - 3 stickers - 3 marbles = 43.90 - 35.40
4 stickers - 3 stickers + 3 marbles - 3 marbles = 8.50
1 sticker = 8.50

Cost of 1 sticker = $8.50

(c)
Substitute 1 sticker = 8.50 into (4).

1 sticker + 1 marble = 11.80
8.50 + 1 marble = 11.80
1 marble = 11.80 - 8.50 = 3.30

Cost of 1 marble = $3.30

Answer(s): (a) $11.80; (b) $8.50; (c) $3.30