3 marbles and 1 sticker cost $18.10.
1 marble and 3 stickers cost $15.10.

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 7 marbles and 7 stickers.

(a)
3 marbles + 1 sticker = 18.10 --- (1)
1 marble + 3 stickers = 15.10 --- (2)

(1) + (2)
4 marbles + 4 stickers = 18.10 + 15.10
4 marbles + 4 stickers = 33.20 --- (3)

(3) _{÷ 4}
4 marbles + 4 stickers = 33.20
1 marble + 1 sticker = 33.20 ÷ 4
1 marble + 1 sticker = 8.30 --- (4)

(b)
(2) - (4)
(1 marble + 3 stickers) - (1 marble + 1 marble) = 15.10 - 8.30
1 marble - 1 marble + 3 stickers - 1 sticker = 6.80
2 stickers = 6.80
1 sticker = 6.80 ÷ 2 = 3.40

Cost of 1 sticker = $3.40

(c)
Substitute 1 sticker = 3.40 into (4).
1 marble + 1 sticker = 8.30
1 marble + 3.40 = 8.30
1 marble = 8.30 - 3.40 = 4.90

Cost of 1 marble = $4.90

(d)
(4) _{x 7}
1 marble + 1 sticker = 8.30
7 x (1 marble + 1 marble) = 7 x 8.30
7 marbles + 7 stickers = 58.10

Cost of 7 marbles and 7 stickers = $58.10

Answer(s): (a) $8.30; (b) $3.40; (c) $4.90; (d) $58.10