Math problem...?

Question

At 7:00 a.m. Joe starts jogging at 6 mi/h. At 7:10 a.m. Ken starts off after him. How fast must Ken run to overtake Joe at 7:30 a.m.?

3 mi/h
or
5 mi/h
or
7 mi/h
or
9 mi/h

thanks

Answer 1

Joe runs for 30 minutes (1/2 hour) at 6 mph.
Ken runs for 20 minutes (1/3 hour) at X mph

They run the same distance:
(1/2)6 = (1/3)X
3 = (1/3)X

Multiply both sides by 3 to cancel the 1/3:
3 * 3 = X
X = 9

Ken runs at 9 miles an hour.