A train travels 180 km at a uniform speed. If the speed had been 9 km/hour more, it would have taken 1 hour less for the same journey. Find the speed of the train.

Given: Distance travelled by the train, d=180km, increased speed of the train=9km/h and less time taken by the train=1 hour.
To do: To find the speed of the train.
Solution: Let us say that the train travels from A to B with a uniform speed s km/h and time taken by the train for the journey is t hours.
As known that $distance=speed\times time$ or $time=\frac{distance}{speed} $ or $t=\frac{d}{s}$
The train travelled $d=180\ km$ 
$\therefore \ t=\frac{180}{s}$                    $\dotsc \dotsc \dotsc \dotsc \dotsc ( 1)$
On increasing its speed 9 km/h, it takes 1 hour less time to reach the same destination, as given in the question
$t-1=\frac{180}{s+9}$
or
$\ t=\frac{180}{s+9} +1$                       $\dotsc \dotsc \dotsc \dotsc \dotsc \dotsc ( 2)$
on comparing $( 1)$ and $( 2)$
$t=\frac{180}{s} =\frac{180}{s+9} +1$
Or
$s( 189+s) =180( s+9)$
$\Rightarrow s^{2} +189s=180s+1620$
$\Rightarrow s^{2} +9s-1620=0$
$\Rightarrow ( s-36)( s+45) =0$
If $s+45=0$
$\Rightarrow s=-45$ 
$\because$ speed can't be negative,we reject this value.
If $s-36=0$
$\Rightarrow s=36$ km/h

$\therefore$ The speed of the train is 36 km/h. 
 

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Updated on: 10-Oct-2022

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