A person runs the first 1/4th of the distance at a speed of 8 km/h, the next 3/5th at a speed of 6 km/h, and the remaining distance at a speed of 10 km/hr. Find his average speed:
a) 6.4 km/h
b) 6.7 km/h
c) 7.2 km/h
d) 7.5 km/h