Average Speed

Young boy named Zhores is to visit his Grandma. For this he travels first by train to the nearest large city, then by the bus to the small village, where Granny resides. Given that train ride takes D days with speed Vt (km/h) and bus route takes H hours with speed Vb - what is the average travel speed, respecting P minutes gap between train arrival and bus departure?

For an answer please provide a formula, based upon the variables given here, e.g.

(Vt + Vb) / 2

Use only those variables which you think are necessary. Specific form of expression is not important - just it should calculate to proper result. The example above is, of course, slightly incorrect :)

Example in numbers, for self-check:

D = 2 days
Vt = 80 km/h
H = 9 hours
Vb = 40 km/h
P = 280

should result in average speed 70 kilometers per hour.