In a homogeneous, non-magnetic, highly insulating and viscous medium, a moving particle experiences a viscous drag given by equation  $\overrightarrow{f}=-b\overrightarrow{\upsilon }$. Here b is a positive constant and  $\overrightarrow{\upsilon }$  is velocity of the particle. A particle having charge q projected with an unknown velocity from a point in the medium almost stops (its speed becomes practically negligible) after travelling a distance of 10 m in a straight line. Now a uniform and constant magnetic field is established in the region and the same particle is again projected with the same velocity directed perpendicular to the magnetic field. 
In the presence of the magnetic field, if the particle practically stops 6 m away from the point of projection, the magnitude of magnetic induction is equal to  $n\frac{b}{q}$. Find n (assume gravity free space)