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