Question

A train travels 20km/h at a uniform speed of 60km/h and next 20km/h at a uniform speed of 80km/h . calculate it's average speed.​

Answer 1

Answer:

68.75km/h

Explanation:

Distance first(D1)=20km

Speed first(S1)=60km/h

So,

Time(T1)=Distance/speed

20/60 = 1/3

Now,

Distance second(D2)=20km

Speed second(S2)=80km/h

So,

Time (T2)=

20/80=1/4

Now,

Total distance=20+20=40km

Total time=

1/3+1/4

4+3/12=7/12

We know that average speed=total distance/total time

40/ 7/12

40x12/7

480/7=68.75.

Hence, average speed=68.57km/h

Hope it helps you!

Thank you!

Answer 2

Answer:

68.965 km/h

Explanation:

As per the provided information in the given question, we have :

• A train travels 20 km at a uniform speed of 60km/h

And, next 20km at a uniform speed of 80km/h.

We are asked to calculate the average speed.

In order to calculate the average speed of the train, firstly we need to find total distance and total time taken by the train.

★ Finding total distance :

$\begin{gathered}\longmapsto \rm {Distance_{(Total)} =20 + 20 }\\ \end{gathered}$

$\begin{gathered}\longmapsto \bf {Distance_{(Total)} = 40 \; km}\\ \end{gathered}$

∴ Total distance covered is 40 km.

$\rule{200}2$

★ Finding total time :

• Let the first 20 km's travelled by the train be XY & The next 20 km's Travelled by the train be YX.

$\begin{gathered}\longmapsto \rm {Time_{(Total)} = Time_{(XY)} + Time_{(YX)} }\\ \end{gathered}$

$\begin{gathered}\longmapsto \rm {Time_{(Total)} = \dfrac{Distance_{(XY)} }{Speed_{(XY)}} + \dfrac{Distance_{(YX)} }{Speed_{(YX)}} }\\ \end{gathered}$

$\begin{gathered}\longmapsto \rm {Time_{(Total)} = \Bigg ( \dfrac{20 }{60} + \dfrac{20 }{80} \Bigg ) \; h }\\ \end{gathered}$

$\begin{gathered}\longmapsto \rm {Time_{(Total)} = \Big ( 0.33 + 0.25\Big ) \; h }\\ \end{gathered}$

$\begin{gathered}\longmapsto \rm {Time_{(Total)} = 0.58\; hr }\\ \end{gathered}$

∴ Total time taken is 0.58 hour.

$\rule{200}2$

★ Finding average speed :

$\begin{gathered} \longmapsto\bf {Speed_{(avg)} = \dfrac{Total \; distance}{Total \; time} }\\ \end{gathered}$

$\begin{gathered} \longmapsto\rm {Speed_{(avg)} = \Big ( \frac{40}{0.58} \Big ) \; kmh^{-1} }\\ \end{gathered}$

$\begin{gathered} \longmapsto\rm {Speed_{(avg)} = 68.965 \; kmh^{-1} }\\ \end{gathered}$

∴ Average speed of the train is 68.965 km/h.