For intensity to be maximum Δx=nλ (n=0,1,2...) ⇒xcosθ=nλ cosθ=
nλ
x
or cosθ≯1 ∴
nλ
x
≯1 ∴n≯
x
λ
Substituting x=5λ n≯5 or n=1,2,3,4.... Therefore, in all four quadrants, there can be 20 maximas. There are more maximas at θ = 0° and θ = 180°. But n = 5 corresponds to θ = 90° and θ = 270°, which are coming only twice. While we have multiplied it four times. Therefore, total numberof maximas still 20