Explanation:

We know that,  Speed = Distance/time

Let the speed of the train be = x kmph
           NORMAL SPEED                      SPEED 5 KMPH MORE
          Distance = 360 Km                         Distance =360
          speed    =  X Kmph                        Speed = (x+5)
            Time =360/X                              Time =360/(x+15)

For the same journey if the speed increased 10 kmph more it will take 1 hour less
Time with original speed - time with increased speed = 1
(360/x) - (360/x+10) =  1                              
(360(x+5)-360x)/x(x+5) =1
360X + 1800 -360X  = X(X+5)
X^2 + 5x - 1800 = 0
X = 40 or -45
X cannot be negetive value         

X = 40 kmph.

Hence, option 4 is the correct answer.