Answer: the cost per unit of gas is $0.28
Explanation:
Let x represent the cost per unit of electricity.
Let y represent the cost per unit of gas.
During one month, a homeowner used 500 units of electricity and 100 units of gas for a total cost of $333. This is expressed as
500x + 100y = 333 - - - -- - -- - -- -1
The next month, 400 units of electricity and 150 units of gas were used for a total cost of $286. This is expressed as
400x + 150y = 286 - - - - - - - - - --2
Multiplying equation 1 by 400 and equation 2 by 500, it becomes
200000x + 40000y = 133200
200000x + 75000y = 143000
Subtracting, it becomes
- 35000y = - 9800
y = - 9800/- 35000
y = 0.28
Substituting y = 0.28 into equation 2, it becomes
400x + 150 × 0.28 = 286
400x = 286 - 42 = 244
x = 244/400 = 0.61