Distance travelled by the train in 3 hrs at 60kmph is 
speed*distance=60*3= 180kms.
180 kms = 180000meters..
Therefore no of poles is 180000/20 =9000

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Distance between two poles = 20m
in three hours at the speed of 60 km/h, 
total distance travelled = 60×3 = 180 km
180 km = 180×1000 m = 1,80,000 m

number of poles in an interval of 20m = 2(one at starting and one at 20m)
number of poles in an interval of 40m = 3(one at starting, one at middle and one at 40m)
It is always 1 greater than (distance/20)

So number of poles = 180000/20 + 1 = 9000+1 = 9001
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