You started on a bike trip, riding at a rate of 9 miles per hour. your friend left one hour later and rode at the rate of 12 miles per hour. how long did it take your friend to catch up with you?

How many revolutions will a car wheel of diameter 28 in. make over a period of half an hour if the car is traveling at 60 mi/hr? the final answer is supposedly 21,609 but i have no idea how to get that, show work : )