Jar Jar starts jogging at a constant rate of five miles per hour. Half an hour later, Darwin starts running along the same route at seven miles per hour. For how many minutes must Darwin run to catch Jar Jar?
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.