Johnny can use the car for one day, leaving him with 100$.
To get the most out of the miles, he would have to spend the rest of the 100$.
You have to go through a list of numbers to get it, I started with 1.5 · 20, which is 30. Then 1.5 · 40 which was 60, then 1.5 · 60 which was 90, then 1.5 · 70 which was 105 (i think), then I went back down to 1.5 · 65 which was 97.5, so then I landed on...
1.50$ · 66 = 99$
Johnny can travel 66 miles for one day with 200$.