Question

A car made a run of 390 km in `x` hours. If the speed had been 4 km/hour more, it would have taken 2 hours less for the journey. Find `x`.

Answer 1

Let the actual speed be v km/hour
actual speed of run,v = distance/time = km/hour
if this speed is increased by 4 km/hour, time will be 2 hours less
i.e

$\Rightarrow 780=v^2+4v$

$\Rightarrow v^2+4v-780=0$

$\Rightarrow v^2+30v-26v-780=0$

$\Rightarrow v(v+30)-26(v+30)=0$

$\Rightarrow (v+30)(v-26)=0$

so, v= 26 km/hours ,

neglecting negative value

so, time, $x=\frac{390}{26}=15$hours