
P and Q are two cities. A boy travels on cycle from P to Q at a speed of 20 km per hr. and returns at the rate of 10 km per hr. Find his average speed for the whole journey.


13 2 kmph 3 
12 1 kmph 3 
13 1 kmph 3 
12 2 kmph 3

Correct Option: C
Let the speed between P and Q be x km.
Then time taken to cover x km. P to Q =  hrs.  
20 
Time taken to cover x km from Q to P at 10 km per hr. P to Q
=  hrs.  
10 
∴ Total distance covered
= x + x = 2x km.
Time taken to cover 2x km.
=  +  =  =  hrs.  
20  10  20  20 
∴ Average Speed =  =  
3x/20  3x 
=  km per hr.  
3 
= 13  km per hr.  
3 
Second Method : Here, x = 20 km per hr.
y = 10 km per hr
∴ Average Speed [∵ Distance is same]
=  =  
x + y  20 + 10 
=  =  = 13  km per hr.  
30  3  3 