5 stickers and 1 marble cost $21.60.
1 sticker and 5 marbles cost $19.20.

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 6 stickers and 6 marbles.

(a)
5 stickers + 1 marble = 21.60 --- (1)
1 sticker + 5 marbles = 19.20 --- (2)

(1) + (2)
6 stickers + 6 marbles = 21.60 + 19.20
6 stickers + 6 marbles = 40.80 --- (3)

(3) _{÷ 6}
6 stickers + 6 marbles = 40.80
1 sticker + 1 marble = 40.80 ÷ 6
1 sticker + 1 marble = 6.80 --- (4)

(b)
(2) - (4)
(1 sticker + 5 marbles) - (1 sticker + 1 marble) = 19.20 - 6.80
1 sticker - 1 sticker + 5 marbles - 1 marble = 12.40
4 marbles = 12.40
1 marble = 12.40 ÷ 4 = 3.10

Cost of 1 marble = $3.10

(c)
Substitute 1 marble = 3.10 into (4).
1 sticker + 1 marble = 6.80
1 sticker + 3.10 = 6.80
1 sticker = 6.80 - 3.10 = 3.70

Cost of 1 sticker = $3.70

(d)
(4) _{x 6}
1 sticker + 1 marble = 6.80
6 x (1 sticker + 1 marble) = 6 x 6.80
6 stickers + 6 marbles = 40.80

Cost of 6 stickers and 6 marbles = $40.80

Answer(s): (a) $6.80; (b) $3.10; (c) $3.70; (d) $40.80