Question

A car travels 30 km with a uniform speed of 30km hr what will be its speed for next 90 km if average sppedd for whole journey is 60 km hr

Answer 1

s=d/t

s=d1+d2/t1+t2

60=30+90/t1+t2

t1=d1/s1=30/30hr
t1=1hr

hence60=120/1+t2

1+t2=2
t2=1

therefore s2=d2/t2
s2=90/1hr
s2=90km/hr

Answer 2

distance = 30 km , speed = 30 km/hr

therefore time = 1 hr (time = distance/speed)

average distance = (total distance/total time taken)

total distance = 90 + 30 = 120 km

total time taken = 1hr + xhr

therefore

120/(1+x) = 60     => x = 1 hr (time for the second journey of 90 km)

therefore

speed = 90/1 = 90km/hr