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How fast did the courier bike, when he delivers a letter on moving column? A military column of 1 km length marches with a constant speed of 6 kilometers

per hour. A courier from the back of the column is sent to the head of the column by bike to deliver a message. Arrived at the front, he instantly returns. When he returns at the back, the column has traveled a distance of 1 km since the start of his errand. The courier drove with constant speed.


Column speed = 6 kmph (given)
Distance travelled by Column = 1 km. (given)
Let the time taken by Column to travel 1 km be ' T ' hr.
speed = distance / time ( Basic formulae )
6 = 1/T 
T = 1/6 hr ( i.e 0.1666667 hr ) ---> (i)

It is given that during this ' T ' hr, courier travelled from back of column to front & from front to back of the column. Since the Column is 1 km long, so the distance travelled by courier is 2 km. 

Finally applying basic formulae of speed to courier i.e. 
speed = distance travelled/ Time taken
= 2 km / 0.166666667 hr ( From (i) )
= 2 / 1/6 
= 12 kmph
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