Question

A car covers 30km at a uniform speed of 30km/hr. What should be its speed for next 90km if the average speed for the entire journey is 60km/hr?

Answer 1

first of all....
d1=30 km
s1=30 km/hr
t1=d/s=30/30=1 hr
 
now ,the total distance=30 km+ 90 km= 120 km
total time =t1+t2=1+t2
avg speed=total distance/total time
            60=120/t1+t2
            60+60t2=120
               60t2=120-60
               60t2=60
                  t2=60/60
                  t2=1 hr
 case 2=
d2=90 km/hr
t2=1 hr
s2=d/t=90/1=90 km/hr