$100 - $49.95 = $50.05
You have $100, and just renting costs $49.95, leaving $50.05 left.
$50.05 / $0.39 = 128.3
Out of your money left ($50.05) you divide my how much it costs in a day, to get how many miles you can go.
The answer is 128.3, but if you were going a round trip (there and back) you would do 128.3 / 2 = 64.16, meaning you can go 64 miles and come back, all for a hundred dollars (still 128 miles in total though)
