To travel 612 km, Train A takes 9 hours more than Train B. If the speed of the Train A is doubled, it takes 3 hours less than Train B. The speed (in km/h) of Train B is:

40.8 km/h

Let us consider that ,

Speed of B = s km/h

We know that ,

Time = \(\frac{Distance}{Speed}\)

Time taken by B = \(\frac{612}{s}\)

Time taken by A = [ ( \(\frac{612}{s}\) ) + 9 ]

Speed of A = \(\frac{612}{ [ 612/s + 9 ]}\) 

According to question ,

612 = 2 × \(\frac{612}{ [ 612/s + 9 ]}\) × [ \(\frac{612}{s}\) - 3 ]

612 = 2 × \(\frac{612}{ [ 612/s + 9 ]}\) × [ \(\frac{612}{s}\) - 3 ]

\(\frac{612}{s}\) + 9 = 2 × [ \(\frac{612}{s}\) - 3 ]

\(\frac{612}{s}\) + 9 = 2 ×  \(\frac{612}{s}\) - 6

\(\frac{612}{s}\) = 15 

s = 40.8 km/h