Thickness of slab

previously,

optical path difference = 0 (at the center f the screen)

after film is kept,

optical path diff = μ X (thickness of slab) = μ t [t = thickness of slab , say]
intial intensity = 4I₀ [I₀ = intensity of each slit]

later, intensity bcomes 75% i.e. 3I₀.

Δθ = Δx X 2pi/λ = 2pi . μt/λ ------------ (1)

intensity = 4I₀cos²(Δθ/2) ----------- (2)

from 1 and 2,

cos (Δθ/2) = ± √3/2
=> (Δθ/2) = pi/6 , 5pi/6 , 7pi/6 , 11pi/6 .

we can clearly c, t is min for pi/6

so ... finally ..

t = λ/6μ