Question

An increase of speed of 4km/h on a journey of 32km reduces the time taken by 4 hours. Find the original speed.

Answer 1

Answer:

Let v be the original speed and t be the time taken in hours to complete the journey

Then,

v = 32 / t

t = 32 / v ………(1)

Also,

v + 4 = 32 / (t - 4)

v + 4 = 32 / [(32 / v) - 4] …………[∵ from equation (1)]

v + 4 = 32 / [(32 - 4v) / v]

v + 4 = 32v / (32 - 4v)

(v + 4) × (32 - 4v) = 32v

32v - 4v² + 128 - 16v = 32v

4v² + 16v - 128 = 0

On solving above equation we will get

v = 4 kmph or v = -8 kmph

Speed can’t be negative.

∴ Original speed is 4 kmph