arithmetic - a train moves past a tunnel 360 m long and a telegraph post in 21 seconds and 9 seconds respectively what is the speed of the train - skillgun

Let the length of the train be x metres and its speed be y m/sec.
then, x/y = 9
=> x = 9y.
Now, (x+360)/21 = y
=> 9y + 360 = 21y
=> 12y = 360
=> y = 30 m/sec.
= (30 *18/5) km/hr
= 108 km/hr.