Speed of a Train

$6s = 6/3600 = \frac{1}{600} hour\ldots(1)$

$150 m = 150/1000 = 0.15 km\ldots(2)$

$v_{total} \times t = distance$

$\Rightarrow(v_{train}+v_{man})\frac{1}{600} = 0.15$

$\Rightarrow(v_{train}+6)\frac{1}{600} = 0.15$

$\Rightarrow v_{train}+6 = (0.15)(600)$

$\Rightarrow v_{train} = (90)-6$

$\Rightarrow v_{train} = \boxed{84}$ km/jam

At first, you need to convert the time into hours and length into km. (As the answer required is in km/hr)

--> 6s=6/3600 hrs= 1/600 hrs

-->150m=3/20km

In this problem, train and man are moving in opposite directions. So the velocities get ADDED UP. (Reverse for things moving in same direction).

Let the velocity of train be x km/hr. Then :

Total velocity=(x+6)km/hr

Distance travelled in six seconds=(x+6)/600 km.

Here, distance travelled= length of train...

So, (x+6)/600=3/20.

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x=84. Speed of train=84 km/hr.