Question 500895
Let Speed of Car A = 40mph (which leaves at 1:00PM)
Let Speed of Car B = 55mph  (which leave at 1:30PM)

Distance the car A would have covered in 30 mins = 40 * 30/60 = 20 miles.

At 1:30, Car A would be 20 miles from San diego and Car B at San diego.
Assume after time t, the distance between the two cars is 15 miles

Distance covered in time t by Car A = 40t + 20miles  (distance covered from 1 to 1:30 PM)

Distance covered in time t by Car B = 55t

40t + 20 - 55t = 15 miles (distance between both cars is 15 miles)
-15t = -5
t = 1/3 hrs
   = 1/3 * 60 = 20 mins

Hence the time is 1:30 + 20 mins = 1:50 PM 
At 1:50PM the distance between two cars will be 15 miles

CHECK:: Distance covered by Car A (which starts at time 1:00PM ) 
= 40*(1/2+1/3) = 40*5/6 = 200/6 = 100/3 miles
Distance covered by car B (starting at 1:30PM) = 55*1/3 = 55/3 miles
Difference in the distance = 100/3 - 55/3 = 45/3 = 15 miles