3 stickers and 1 bead cost $12.80.
1 sticker and 3 beads cost $8.80.

1. Find the cost of 1 sticker and 1 bead.
2. Find the cost of 1 bead.
3. Find the cost of 1 sticker.
4. Find the cost of 4 stickers and 4 beads.

(a)
3 stickers + 1 bead = 12.80 --- (1)
1 sticker + 3 beads = 8.80 --- (2)

(1) + (2)
4 stickers + 4 beads = 12.80 + 8.80
4 stickers + 4 beads = 21.60 --- (3)

(3) _{÷ 4}
4 stickers + 4 beads = 21.60
1 sticker + 1 bead = 21.60 ÷ 4
1 sticker + 1 bead = 5.40 --- (4)

(b)
(2) - (4)
(1 sticker + 3 beads) - (1 sticker + 1 marble) = 8.80 - 5.40
1 sticker - 1 sticker + 3 beads - 1 bead = 3.40
2 beads = 3.40
1 bead = 3.40 ÷ 2 = 1.70

Cost of 1 bead = $1.70

(c)
Substitute 1 bead = 1.70 into (4).
1 sticker + 1 bead = 5.40
1 sticker + 1.70 = 5.40
1 sticker = 5.40 - 1.70 = 3.70

Cost of 1 sticker = $3.70

(d)
(4) _{x 4}
1 sticker + 1 bead = 5.40
4 x (1 sticker + 1 marble) = 4 x 5.40
4 stickers + 4 beads = 21.60

Cost of 4 stickers and 4 beads = $21.60

Answer(s): (a) $5.40; (b) $1.70; (c) $3.70; (d) $21.60