Question

a car cover 30 km at a uniform speed of 30 km/hr .what should be its speed for the 90 km if the average speed for the entire journey is 60 km/hr

Answer 1

Total distance = 30 + 90 = 120 km

time for 30 km = 30/30 = 1 hr

let v be the speed for 90 km

so, time for 90 km = 90/v h

Total time = 1 + 90/v = (v + 90)/v

As average speed = total distance/total time

so , 60 = 120/{(v + 90)/v}

or, (v + 90)/v = 2

or, (v + 90) = 2v

or, v = 90 km / h.

So, speed for the 90 km should be 90 km/h

Answer 2

Total distance = 30 + 90 = 120 km

time for 30 km = 30/30 = 1 hr

let v be the speed for 90 km

so, time for 90 km = 90/v h

Total time = 1 + 90/v = (v + 90)/v

As average speed = total distance/total time

so , 60 = 120/{(v + 90)/v}

or, (v + 90)/v = 2

or, (v + 90) = 2v

or, v = 90 km / h.

So, speed for the 90 km should be 90 km/h
ANSWER