time

Question does not mention one important point,weather A and B started from same point and moved towards each other or away from each other.
If I assume A and B started from same point and moved away from each other,
At 9pm distance between them 3*20=60 miles
from 9pm relative speed is 40+20=60,iles per hour
so they will be 240miles apart in another (240-60)/60=3hr or at 12am

d=rt
since b starts 3 hrs earlier than a, t=t+3
since a starts 3 hrs after b, t=t
For B,
x=20(t+3)

For A,
240-x=40t

240 -[20(t+3)]=40t
t=3 hrs
so 3 hrs from when A leaves = time to 240 mi apart
or
6 hrs (t+3) from when B leaves= time to 240 mi apart
=12a