18. A posotivley charged particle falling freely under gravity enters a region having a uniform horizontal magnetc field pointing north to south. The paticle will be deflected towards which direction?

Towards west. You can show that using Fleming right hand rule again or unit vectors.

velocity of particle is along negative x-axis denoted by -i

magnetic field along negative z-axis denoted by -k

Therefore direction of force is along (-i x -k) = i x k = -j

-j denotes west