how to find the no: of fringes????????????????pls explain

as we know from the basic theory, the distance of screen from the slits is D, the point P is at a distance of y from the central maxima, Hence

y= nλD/d

and d is the sepearation between two slits

if the slita are moving apart with the realtive velocity of v

so the separation d is changing with time as v

d = vt

Hence, y= nλD/vt

rearrage them, we get n/t = yv/λD.

Hope it is clear.