1.  The ratio between the speed of two trains is 7 : 8. If the second train runs 400 kms 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

Answer: Option D

Explanation:

Let the speeds of two trains be 7x and 8x km / hr.
Then, 8x = 400/4 = 100
=> x = (100/8) = 12.5

Speed of first train = (7 x 12.5 ) km / hr
                           = 87.5 km / hr