Question

a car cover total journey in two equal interval with speed 20km/h and 40km/h then it s average speed is ​ correct answer

Answer 1

Answer:

Option - (A) - 80/3 km/hr

Explanation:

Total journey distance = D,

                      but it is divided into 2 equal intervals. So both intervals are D/2 and D/2

We have 2 time lapse 1st in the 1st d/2=T1 lap and

                              2nd in the 2nd d/2=T2 lap that is

Total time taken = T1 lap + T2 lap

                Now, T1 = ?     and     T2 = ?

So, T1 = $\frac{d/2}{20}$ = d/40 and

      T2 = $\frac{d/2}{40}$ = d/80

                       So, Total time taken = T1 + T2    

                                                          =>    d/40 + d/80  

Average speed = Total distance/Total time taken

                           =>  $\frac{D }{d/40 + d/80}$             => $\frac{D}{2d + d/ 80}$

                            =>  $\frac{D}{3d/80}$                    => 80/3 km/hr

Answer 2

Given : Car travel in two equal half distance

            Speed of first half =20 km/hr

            Speed of second half = 40 km/hr

To Find : Average speed of the car

Explanation:

Let consider, x= total distance covered by the car,

Time taken to complete the half distance with velocity is 20km/h,

$\[{t_1} = \frac{x}{{2 \times 20}} = \frac{x}{{40}}\]$

The time taken to complete another half distance with velocity is 40km/h.

$\[{t_2} = \frac{x}{{2 \times 40}} = \frac{x}{{80}}\]$

The total time taken, t = $\[{t_1}\]$+$\[{t_2}\]$

$\[t = \frac{x}{{40}} + \frac{x}{{80}} = \frac{{3x}}{{80}}\]$ hr

The average speed of the car,

$\[{v_{avg}} = \frac{x}{{3x/80}}\]$ km/hr

$\[{v_{avg}} = \frac{{80}}{3}\]$ km/hr

Hence the average speed of the car is $\[\frac{{80}}{3}\]$ km/hr.