A train covers a distance in 50 minutes, if it runs at a speed of 48 km/hr on an average. Find the speed at which the train must run to reduce the time of journey to 40 minutes.

The correct option is B 60 km/hr
We are given time and speed, so first we will calculate the distance. Then we can get new speed for given time and distance.

Time = $\frac{50}{60}$ hr = $\frac{5}{6}$ hr
Speed = 48 km/hr
Distance = Speed × Time = $48\times \frac{5}{6}$ = 40 km

New time = 40 minutes = $\frac{40}{60}$ = $\frac{2}{3}$ hr

Now we know,
Speed = $\frac{Distance}{Time}$.
New speed = $\frac{40}{\left(\frac{2}{3}\right)}$ = $\frac{40\times 3}{2}$ = $\frac{120}{2}$ = 60km/hr .