Question

A car moving on a straight road covers the one fourth of the total distance with 36
km/h and the rest with 54 km/h, the average speed of car is​

Answer 1

$\underline{\mathrm{Answer}} : \\ \\ \mathrm{Average\ speed\ = \dfrac{Total\ Distance\ covered}{Total\ Time\ taken}} \\ \\ \\ \mathrm{Let\ the\ total\ distance\ covered\ be\ 'x'\ .}\\ \\ \\ \mathrm{Time\ taken = \dfrac{Distance}{Speed}} \\ \\ \\ \mathrm{Time\ for\ first\ part= \dfrac{\dfrac{x}{4}}{36} = \dfrac{x}{144}\ sec} \\ \\ \\ \mathrm{Time\ for\ second\ part = \dfrac{\dfrac{3x}{4}}{54} = \dfrac{x}{72} \ sec = \dfrac{2x}{144}\ sec}$

$\mathrm{Total\ time\ taken\ = \dfrac{x}{144} +\dfrac{2x}{144} = \dfrac{3x}{144} = \dfrac{x}{48}} \\ \\ \\ \mathrm{Average\ speed = \dfrac{x}{\dfrac{x}{48}}=48\ km/hr } \\ \\ \\ \boxed{\mathrm{Therefore\ average\ speed\ of\ the\ car\ is: \ 48 km/hr}}\\ \\ \\ \mathrm{Hope\ this\ helps :\ )}$

Answer 2

Given : A car moving on a straight road covers the one-fourth of the total distance with 36 km/h and the rest with 54 km/h,

To Find : the average speed of the car  

Solution:

Let say total Distance = 4D km

one-fourth of the total distance = D km

Speed = 36 km/hr

Time = D/36   hr

rest distance = 4D - D = 3D km

Speed = 54 km/hr

Time = 3D/54  = D/18   hr

Total Time = D/36  + D/18

= (3D)/36

= D/12

Average speed  = 4D/(D/12)   =  48 km/hr

Average speed   of  the Car = 48 km/hr

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