Question

He covers one-fourth of the total distance at 20 km/hr, one-fourth at 10 km/h and rest of his journey at 80 km/h. Find Rohit’s average speed for the whole distance?

Answer 1

Answer:

Let the dstance be 400 km

Time taken to cover first one-fourth

= 100/20 = 5 h

Time taken to second one-fourth = 100/10 = 10h

Time taken to rest of the distance = 200/80 = 2.5h

Average speed = 400/17.5

= 22.8km/h ans.

Answer 2

The average speed of Rohit is 22.8 km/h.

Step-by-step explanation:

Let the total distance travelled by Rohit be x km.

1. One-fourth of x is travelled at a speed of 20 km/h,

So, time taken to cover this distance = $\frac{distance}{speed}$ = $\frac{x/4}{20}$ = $\frac{x}{80}$ h.

2. One-fourth of x is travelled at a speed of 10 km/h,

So, time taken to cover this distance = $\frac{distance}{speed}$ = $\frac{x/4}{10}$ = $\frac{x}{40}$h.

3. Rest of the distance,

i.e. $x- (\frac{x}{4} +\frac{x}{4})$ = $\frac{x}{2}$ is travelled at a speed of 80 km/h,

So, time taken to cover this distance = $\frac{distance}{speed}$ = $\frac{x/2}{80}$ = $\frac{x}{160}$ h.

Total distance = x km

Total time taken = $\frac{x}{80} +\frac{x}{40} + \frac{x}{160}$ h = $\frac{(2x+ 4x+x)}{160}$h = $\frac{7x}{160}$ h

Thus, average speed = $\frac{distance}{time}$ = $\frac{x}{(7x/160)}$ = $\frac{160}{7}$ = 22.8 km/h.