One month, a homeowner used 150 units of gas and 520 units of electric for a total cost of $84.20. The next month, 210 units of gas and 405 units of electric were used for a total cost of $82.35. Find the cost per unit of gas and electric using substitution.

Let g represent the cost of gas and e represent the cost of electricity.

150g + 520e = 84.20 ⇒ 7 (150g + 520e = 84.20) ⇒ 1050g + 3640e = 576.80

210g + 405e = 82.35 ⇒ - 5 (210g + 405e = 82.35) ⇒ - 1050g - 2025e = - 411.75

1615e = 165.05

e = 0.10

150g + 520 (0.10) = 84.20

150g + 52 = 84.20

140g = 32.20

g = 0.23

Answer: gas = $0.23, electricity = $0.10