Question

A car travels from one town to the other with average speed 20 km/hr. If the first half is travelled at average speed
30 km/hr., then the average speed of the car in the other half will be:
(A) 30 km/hr
(B) 40 km/hr
(C) 50 km/hr
(D) 15 km/hr ​

Answer 1

the answer will be 15 km/hr

Answer 2

Answer:

$\boxed{\sf The\: average\: speed \:of \:the\: car\: in\: the\: other\: half\: of \: journey = 15 km/hr }$

Explanation:

Solution

Given that , Average Speed = 20 km/ hr.

Let the distance Traveled in the first half of the journey be , 'x'.

We Know that , speed = distance

                                           time

So , time = distance    

                       speed

time =       x      

                   30

Now , let the speed of the second half of the journey be 'u'.

∴ time =        x        

                         u

Here, ∵ the first half and the second half are same distance

Total distance = x + x = 2x

Average Speed =                  Total Distance              

                                                      Total Time

                         

20  =                    2x                  

             ( x/30+x/u )

20 =                   2x                    

          ( x/30 + x / u)

20 =             2              ×       x      

                 ( 30+u)                   30

20 =             2          ×              x        

                (30+u)                      30

20(30+u) = 60 u

600+ 20 u = 60u

600 = 40u

u = $\underline{\underline{{15 km/hr}}}$