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Q.
A car moving on a straight road covers $ 1/3 $ of the distance with $ 25\,km/h $ and rest with $ 75\,km/h $ . The average speed is
J & K CETJ & K CET 2014Motion in a Straight Line
Solution:
$=V_{a V}=\frac{\text { Total distance covered }}{\text { Total time taken }} $
$=\frac{\frac{x}{3}+\frac{2 x}{3}}{t_{1}+t_{2}}$
[Let the total distance covered is $x\, km$ ]
$=\frac{\frac{x}{3}+\frac{2 x}{3}}{\frac{\left(\frac{x}{3}\right)}{25}+\frac{\left(\frac{2 x}{3}\right)}{75}}$
$=\frac{x}{\frac{x}{75}+\frac{2 x}{225}}=\frac{x}{\frac{(3+2) x}{225}} $
$=\frac{225}{5}=45\, m / s$