Question 242723
so write down what we know:

e = electric    g = gas

We know that in one month:  (1)400e + 150g = $76.50
and in the next month:      (2)450e + 100g = $78.50

So we need to use elimination to solve this problem.

Let's eliminate the g..to do that we need to make the units of g's exact opposites, so that when we add the equations we will get zero for the g.

The easiest way to do that is multiple equation 2 by -1.5
this will give us:

-675e - 150g = -117.75
now add this equation to equation 1

 400e + 150g =   $76.50
-675e - 150g = -$117.50

the answer is -275e = -$41.25

divide both sides by -275 and you get e = $0.15

So one unit of electricity is $0.15