Answer in Optics for Kundan #87330
The condition for interference is
where t is the thickness of the air wedge at a distance x from the point of contact of the glass plates.
θ is the angle between the plates in radians (this angle is small, so tan θ = θ in radians). In this case, we got
20” is equal to 9.59×10-5 radians
Using (4) we get 3.29 fringes/cm