2 marbles and 1 sticker cost $11.20.
1 marble and 2 stickers cost $7.70.

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 6 marbles and 6 stickers.

(a)
2 marbles + 1 sticker = 11.20 --- (1)
1 marble + 2 stickers = 7.70 --- (2)

(1) + (2)
3 marbles + 3 stickers = 11.20 + 7.70
3 marbles + 3 stickers = 18.90 --- (3)

(3) _{÷ 3}
3 marbles + 3 stickers = 18.90
1 marble + 1 sticker = 18.90 ÷ 3 
1 marble + 1 sticker = 6.30 --- (4)

Cost of 1 marble and 1 sticker = $6.30

(b)
(2) - (4)
(1 marble + 2 stickers) - (1 marble + 1 marble) = 7.70 - 6.30
1 marble - 1 marble + 2 stickers - 1 sticker = 1.40
1 sticker = 1.40

Cost of 1 sticker = $1.40

(c)
Substitute 1 sticker = 1.40 into (4).
1 marble + 1 sticker = 6.30
1 marble + 1.40 = 6.30
1 marble = 6.30 - 1.40 = 4.90

Cost of 1 marble = $4.90

(d)
(4) _{x 6}
1 marble + 1 sticker = 6.30
6 x (1 marble + 1 marble) = 6 x 6.30
6 marbles + 6 stickers = 37.80

Cost of 6 marbles and 6 stickers = $37.80

Answer(s): (a) $6.30; (b) $1.40; (c) $4.90; (d) $37.80