3 marbles and 1 bead cost $7.70.
1 marble and 3 beads cost $8.70.

1. Find the cost of 1 marble and 1 bead.
2. Find the cost of 1 bead.
3. Find the cost of 1 marble.
4. Find the cost of 7 marbles and 7 beads.

(a)
3 marbles + 1 bead = 7.70 --- (1)
1 marble + 3 beads = 8.70 --- (2)

(1) + (2)
4 marbles + 4 beads = 7.70 + 8.70
4 marbles + 4 beads = 16.40 --- (3)

(3) _{÷ 4}
4 marbles + 4 beads = 16.40
1 marble + 1 bead = 16.40 ÷ 4
1 marble + 1 bead = 4.10 --- (4)

(b)
(2) - (4)
(1 marble + 3 beads) - (1 marble + 1 marble) = 8.70 - 4.10
1 marble - 1 marble + 3 beads - 1 bead = 4.60
2 beads = 4.60
1 bead = 4.60 ÷ 2 = 2.30

Cost of 1 bead = $2.30

(c)
Substitute 1 bead = 2.30 into (4).
1 marble + 1 bead = 4.10
1 marble + 2.30 = 4.10
1 marble = 4.10 - 2.30 = 1.80

Cost of 1 marble = $1.80

(d)
(4) _{x 7}
1 marble + 1 bead = 4.10
7 x (1 marble + 1 marble) = 7 x 4.10
7 marbles + 7 beads = 28.70

Cost of 7 marbles and 7 beads = $28.70

Answer(s): (a) $4.10; (b) $2.30; (c) $1.80; (d) $28.70