5 marbles and 1 sticker cost $8.10.
1 marble and 5 stickers cost $11.70.

1. Find the cost of 1 marble and 1 sticker.
2. Find the cost of 1 sticker.
3. Find the cost of 1 marble.
4. Find the cost of 5 marbles and 5 stickers.

(a)
5 marbles + 1 sticker = 8.10 --- (1)
1 marble + 5 stickers = 11.70 --- (2)

(1) + (2)
6 marbles + 6 stickers = 8.10 + 11.70
6 marbles + 6 stickers = 19.80 --- (3)

(3) _{÷ 6}
6 marbles + 6 stickers = 19.80
1 marble + 1 sticker = 19.80 ÷ 6
1 marble + 1 sticker = 3.30 --- (4)

(b)
(2) - (4)
(1 marble + 5 stickers) - (1 marble + 1 marble) = 11.70 - 3.30
1 marble - 1 marble + 5 stickers - 1 sticker = 8.40
4 stickers = 8.40
1 sticker = 8.40 ÷ 4 = 2.10

Cost of 1 sticker = $2.10

(c)
Substitute 1 sticker = 2.10 into (4).
1 marble + 1 sticker = 3.30
1 marble + 2.10 = 3.30
1 marble = 3.30 - 2.10 = 1.20

Cost of 1 marble = $1.20

(d)
(4) _{x 5}
1 marble + 1 sticker = 3.30
5 x (1 marble + 1 marble) = 5 x 3.30
5 marbles + 5 stickers = 16.50

Cost of 5 marbles and 5 stickers = $16.50

Answer(s): (a) $3.30; (b) $2.10; (c) $1.20; (d) $16.50