Karina uses the system of equations below to compare the monthly utility costs in July and December for electricity, x, and natural gas, y. 750x + 17y = 141.61300x + 30y = 75.90

The equations are 750x + 17y = 141.61 and 300x + 30y = 75.90. Multiply the second equation by 2.5 to get 750x + 75y = 189.75, then subtract the first equation; this gives 58y = 48.14. Dividing by 58 gives y = $0.83 per unit, so the cost of natural gas is $0.83 per unit.

AI-written and checked against the verified answer.