Motion in a straight line

A man walks on a straight road from
his home to a market 2.5 km away with
a speed of 5 km h–1. Finding the
market closed, he instantly turns and
walks back home with a speed of 7.5
km h–1. What is the
(a) magnitude of average velocity, and
(b) average speed of the man over the
interval of time (i) 0 to 30 min, (ii)
0 to 50 min, (iii) 0 to 40 min ?
[Note: You will appreciate from this
exercise why it is better to define
average speed as total path length
divided by time, and not as
magnitude of average velocity. You
would not like to tell the tired man
on his return home that his
average speed was zero !]