Question 564187
You were asked
Write an expression that gives the total cost of n kwh of electricity.
C=7.50+0.09n is a good answer
You were also asked
Write the expression that gives the average cost per kwh.
C(kwh)=C/n is OK, but it has two problems.
An equivalent expression was expected, and using the same letter C for the total cost and the cost per kwh is confusing.
I will use K for the average cost per kwh, and
{{{K=C/n=(7.50+0.09n)/n=7.5/n+0.09}}}
{{{K=7.5/n+0.09}}} was the expression expected because it gives you a single expression as a function of n (no intermediate calculations).
That is expected in algebra and calculus.
I also know that was the intended answer, because all the answers to the final questions seem to come directly from there.
1) As the number of kwh increases, what happens to the average cost of the kwh? You correctly concluded that
"As the number of kwh increases the average cost of the kwh decreases."
You could conclude that from the results of the calculations
2) Would the average cost ever fall below $0.09?
Looking at {{{K=7.5/n+0.09}}} as a function of n, with n being a positive real number, you see that the answer is NO.
{{{K=7.5/n+0.09>0.09}}} because {{{7.5/n>0}}}
If so, identify a value that supports your answer. Not aplicable, because it was not so.
If not, explain how you know.
{{{n>0}}}, so {{{7.5/n>0}}}, and  {{{K=7.5/n+0.09>0.09}}}
3) How many kwh should be purchased for the average cost to be $0.12 per kwh?
{{{7.5/n+0.09=0.12}}} --> {{{7.5/n=0.12-0.09}}} --> {{{7.5/n=0.03}}} --> {{{7.5=0.03n}}} --> {{{n=7.5/0.03}}} --> {{{n=250}}}