1 bag and 2 pencil boxes cost $d. 2 bags, 4 pencil boxes and 4 erasers cost $28. (a) How much does one eraser cost? Express your answer in terms of d.

(b) If d = 13, find the cost of an eraser.

(a) x + 2y = d

2x + 4y + 4z = 28

2(x + 2y = d) = 2x + 4y = 2d

2d + 4z = 28

4z = 28 - 2d

z = 7 - 1/2d

Answer: $7 - 1/2d

(b) 7 - 1/2 (13) = 0.5

Answer: $0.5