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2014-05-12T22:49:26+05:30
Let total distance be x 
according to question x / 2 distance cover in time t1 ,so
x / 2 = 30*t1 ...............(1)
x / 2 distance cover in time t2 ,so
x / 2 = 70*t2.................(2)
average speed = total distance / total time
                      = x / t             ( where t = t1 + t2)
multiply by 7 / 3 in equation (1) 
7x / 6 = 70*t1..................(3)
add equation (2) and (3) we get ,
10x / 6 = (t1 + t2)*70
x / (t1 + t2) = 42 
hence average speed = 42 Km / hr
2 5 2
we can use the shortcut formula , when body travels equal distances with different velocities the average velocity is given by 2*(product of velocities) / sum of velocities