2012-06-23 Sat - The Maths Geek Ride 0001.5 - London to Bletchley to London

In the maths theme;

Two groups (A & B) have the same start point.
They set off in the same direction.
A is traveling at 15mph with a break of 5 minutes every hour.
B is travelling at 20mph with out pauses.

How much time must pass between A and B departures, to allow B to catch A after X?

Caveats:
1] Where X is equal to the exact distance of route yet to be committed to paper print pixels.
2] Assuming that there are no unexpected stoppages for either A or B

Well, if A is travelling at 15mph + 5 minutes stoppage/hr that means that the average speed should actually be 13.75mph and should complete the distance in about 4 hours and 20 minutes. Group be averaging a solid 20mph should make it in about 3 hours flat, so I guess 80 minutes difference should be enough to ensure both groups arrive simultaneously.