2 stickers and 1 marble cost $5.70.
1 sticker and 2 marbles cost $7.80.

1. Find the cost of 1 sticker and 1 marble.
2. Find the cost of 1 marble.
3. Find the cost of 1 sticker.
4. Find the cost of 9 stickers and 9 marbles.

(a)
2 stickers + 1 marble = 5.70 --- (1)
1 sticker + 2 marbles = 7.80 --- (2)

(1) + (2)
3 stickers + 3 marbles = 5.70 + 7.80
3 stickers + 3 marbles = 13.50 --- (3)

(3) _{÷ 3}
3 stickers + 3 marbles = 13.50
1 sticker + 1 marble = 13.50 ÷ 3 
1 sticker + 1 marble = 4.50 --- (4)

Cost of 1 sticker and 1 marble = $4.50

(b)
(2) - (4)
(1 sticker + 2 marbles) - (1 sticker + 1 marble) = 7.80 - 4.50
1 sticker - 1 sticker + 2 marbles - 1 marble = 3.30
1 marble = 3.30

Cost of 1 marble = $3.30

(c)
Substitute 1 marble = 3.30 into (4).
1 sticker + 1 marble = 4.50
1 sticker + 3.30 = 4.50
1 sticker = 4.50 - 3.30 = 1.20

Cost of 1 sticker = $1.20

(d)
(4) _{x 9}
1 sticker + 1 marble = 4.50
9 x (1 sticker + 1 marble) = 9 x 4.50
9 stickers + 9 marbles = 40.50

Cost of 9 stickers and 9 marbles = $40.50

Answer(s): (a) $4.50; (b) $3.30; (c) $1.20; (d) $40.50