Question 262186
We will use the equation d=vt, where d=distance, v=velocity (or speed), and t=time.

Chris: 
d = 3 + 3 = 6 miles
v = 4 mph
t = ?

d=vt
6=4*t
t=6/4
t=1.5 hours

John:
d = 3 + 3 = 6 miles
v = 6 mph
t = ?

d=vt
6=6*t
t=6/6
t=1 hour

Chris started at 7:00am, so he will end at 8:30am.
John started at 7:10am, so he will end at 8:10am.

20 minutes will elapse after John finishes.