3 stickers and 1 marble cost $17.10.
1 sticker and 3 marbles cost $15.30.

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)
3 stickers + 1 marble = 17.10 --- (1)
1 sticker + 3 marbles = 15.30 --- (2)

(1) + (2)
4 stickers + 4 marbles = 17.10 + 15.30
4 stickers + 4 marbles = 32.40 --- (3)

(3) _{÷ 4}
4 stickers + 4 marbles = 32.40
1 sticker + 1 marble = 32.40 ÷ 4
1 sticker + 1 marble = 8.10 --- (4)

(b)
(2) - (4)
(1 sticker + 3 marbles) - (1 sticker + 1 marble) = 15.30 - 8.10
1 sticker - 1 sticker + 3 marbles - 1 marble = 7.20
2 marbles = 7.20
1 marble = 7.20 ÷ 2 = 3.60

Cost of 1 marble = $3.60

(c)
Substitute 1 marble = 3.60 into (4).
1 sticker + 1 marble = 8.10
1 sticker + 3.60 = 8.10
1 sticker = 8.10 - 3.60 = 4.50

Cost of 1 sticker = $4.50

(d)
(4) _{x 9}
1 sticker + 1 marble = 8.10
9 x (1 sticker + 1 marble) = 9 x 8.10
9 stickers + 9 marbles = 72.90

Cost of 9 stickers and 9 marbles = $72.90

Answer(s): (a) $8.10; (b) $3.60; (c) $4.50; (d) $72.90