The ratio between the speeds of two trains is 7 : 8. If the second train runs 400 km in 4 hours, then the speed of the first train is:

A. 70 km/hr

B. 75 km/hr

C. 84 km/hr

D. 87.5 km/hr

Correct Answer :

D. 87.5 km/hr

Let the speed of two trains be 7x and 8x km/hr. Then,$8x=\left(\frac{400}{4}\right)=100$ $\Rightarrow x=\left(\frac{100}{8}\right)=12.5$ $\therefore Speedoffirsttrain=(7x12.5)km/hr=87.5km/hr.$