So Jar already ran 2.5 miles.
1 hour after darwin starts jar is 7.5 miles, darwin is 7.
jar runs 5/60 miles an hour, and darwin runs 7/60 miles an hour.
so 0.5 + 5x/60 = 7x/60.
0.5 = 7x-5x / 60 = 2x/60 = x/30
x = 0.5 * 30 = 15
Darwin takes 1 hour 15 to catch up
If you don't understand anything, feel free to ask.