Question

a car covers (3/5) th distance at the speed of 72 km /h an the rest of distance with 54 km/h the average speed of the car for complete journey is​

Answer 1

Solution :

Let the total distance covered by the car be d

From point A to B

• Speed (s)= 72 km /hr = 72 x 5 / 18 = 20 m/s

• Distance(d) = 3d / 5

We know that ,

➝ s = d / t

On rearranging ,

➝ t = d / s

➝ t = 3d / 5 x 20

➝ t = 3d / 100

From point B to C

• Speed (s)= 54 km /hr = 54 x 5 / 18 = 15 m/s

• Distance(d) = d- 3d / 5 = 2d / 5

We know that ,

➝ s = d / t

On rearranging ,

➝ t = d / s

➝ t = 2d / 5 x 15

➝ t = 2d / 75

$\implies\mathtt{Avg. speed = \dfrac{Total \:distance} {total \:time }}$

$\implies\mathtt{Avg. speed = \dfrac{d} {\dfrac{3d}{100}+ \dfrac{2d}{75} }}$

$\implies\mathtt{Avg. speed = \dfrac{1} {\dfrac{3}{100}+ \dfrac{2}{75} }}$

$\implies\mathtt{Avg. speed = \dfrac{1} {\dfrac{3x3+2x4}{300} }}$

$\implies\mathtt{Avg. speed = \dfrac{1} {\dfrac{17}{300} }}$

$\implies\mathtt{Avg. speed = \dfrac{300}{17} }$

$\implies\mathtt{Avg. speed = 17.64\dfrac{m}{s} }$

The average speed of the car is 17.64 m /s