4 beads and 1 marble cost $18.90.
1 bead and 4 marbles cost $18.60.

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

(a)
4 beads + 1 marble = 18.90 --- (1)
1 bead + 4 marbles = 18.60 --- (2)

(1) + (2)
5 beads + 5 marbles = 18.90 + 18.60
5 beads + 5 marbles = 37.50 --- (3)

(3) _{÷ 5}
5 beads + 5 marbles = 37.50
1 bead + 1 marble = 37.50 ÷ 5
1 bead + 1 marble = 7.50 --- (4)

(b)
(2) - (4)
(1 bead + 4 marbles) - (1 bead + 1 marble) = 18.60 - 7.50
1 bead - 1 bead + 4 marbles - 1 marble = 11.10
3 marbles = 11.10
1 marble = 11.10 ÷ 3 = 3.70

Cost of 1 marble = $3.70

(c)
Substitute 1 marble = 3.70 into (4).
1 bead + 1 marble = 7.50
1 bead + 3.70 = 7.50
1 bead = 7.50 - 3.70 = 3.80

Cost of 1 bead = $3.80

(d)
(4) _{x 7}
1 bead + 1 marble = 7.50
7 x (1 bead + 1 marble) = 7 x 7.50
7 beads + 7 marbles = 52.50

Cost of 7 beads and 7 marbles = $52.50

Answer(s): (a) $7.50; (b) $3.70; (c) $3.80; (d) $52.50