The only graph shown in the question doesn't have amplitude 1/2. look for a graph of a periodic wave function that has maximum y-value 1/2 (0.5) and minimum y-value 1/2 (0.5), or if it is not oscillating around the x-axis, verifies that the distance between minimum y-value and maximum y-value is "1" (one). This is because the amplitude is half of the peak-to-peak distance.
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.