If a car covers 1/3 of the distance at 20 km/h and rest at 60 km/h, then what is its average speed?

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There is no answer. This is a badly posed question. The answer depends upon the total distance driven which we are not given.

 

Suppose the total distance were 300 km. Then 1/3, 100 km, is driven at 20 km/h so takes 100/20= 5 hours. The remainder, 200 km, is driven at 60 km/h so takes 200/60= 3 and 1/3 hours. The entire 300 km was driven in 8 and 1/3 hours so the average speed was 300 km divided by 8 and 1/3 hours or 300/(25/3)= 900/25= 35 km/h.

 

Now, suppose the total distance was 3 km. Then 1/3, 1 km, is driven at 20 km/h so takes 1/20= 0.05 hours. The remainder, 2 km, is driven at 60 km/h so takes 1/60= 0.0167 hours. The entire 3 km was driven in 0.0667 hours so the average speed was 1/0.667= 15 km/h.

Average speed is equal to total speed by total time.

We can assume the total distance as d in the above question and proceed.

I think Let S be the whole distance,

Time for the First coverage=(S/3)/20=S/60

Time for the Second coverage=(2S/3)/60=S/90

So, Average Speed = S/(S/60+S/90)=36km/h

 

On 7/21/2017 at 9:12 AM, HallsofIvy said:

There is no answer. This is a badly posed question. The answer depends upon the total distance driven which we are not given.

 

Suppose the total distance were 300 km. Then 1/3, 100 km, is driven at 20 km/h so takes 100/20= 5 hours. The remainder, 200 km, is driven at 60 km/h so takes 200/60= 3 and 1/3 hours. The entire 300 km was driven in 8 and 1/3 hours so the average speed was 300 km divided by 8 and 1/3 hours or 300/(25/3)= 900/25= 35 km/h.

 

Now, suppose the total distance was 3 km. Then 1/3, 1 km, is driven at 20 km/h so takes 1/20= 0.05 hours. The remainder, 2 km, is driven at 60 km/h so takes 1/60= 0.0167 hours. The entire 3 km was driven in 0.0667 hours so the average speed was 1/0.667= 15 km/h.

You should check your assumptions and math. If you do it correctly you will get the same answer for each distance. The bolded is correct so you almost got the first one correct.

Edited July 30, 20178 yr by J.C.MacSwell