Question

A train covers a distance of 480 km at a speed of $x$ km/hr. If its speed is reduced to $(x-20)$ km/hr, it takes $4$ hours more to cover the same distance. Find the value of $x$.

• A. $47$
• B. $60$
• C. $72$
• D. $80$

Answer 1

The correct option is B $60$

Let the time taken to cover $480$ Km be time, $t$
Then, $xt=480$,

$\therefore t=\frac{480}{x}$

And, $(x-20)(t+4)=480$

$(x-20)(\frac{480}{x}+4)=480$

$(x-20)(480+4x)=480x$
$480x-9600+4x^2-80x=480x$
$4x^2-80x-9600=0$
$x^2-20x-2400=0$
$x^2-60x+40x-2400=0$
$(x-60)(x+40)=0$
$x=60,-40$
Neglect the negative value,
$x=60$ km/hr