Ok so we know he used over 700 because he was charged more than 40. Then we take the $8.10 and divide 0.45 into it. This then gives you 18 more minutes. Then add 18 to 700 and you get 718 min.
N.b., this is copied from an answer I wrote for someone else.Thinking about the graph of y, the rate of change is zero whenever there is an extremum.
First, differentiate y.

Next, find the zeros of y' by factoring.

Now, we substitute these x-values into the
original equation to find the coordinates.
Subtracting a value from the exponent shifts the graph to the right.
So if you subtract 3, the graph would shift 3 units to the right.