To solve this problem we apply the concepts related to the concept of destructive interference. For the destructive interference for a single slit path difference is,
[tex]dsin\theta = n\lambda[/tex]
Where,
D = Slit width
n = Order of the minima
[tex]\theta[/tex] = Angle relative to the original direction of the light
[tex]\lambda[/tex] = Wavelength of the light
Now the first minima will have a n=1 then
[tex]Dsin\theta_1 = 1\lambda[/tex]
And the 5 order fringe will have n=5 then
[tex]dsin\theta_2 = 5\lambda[/tex]
The ratio between the two variables will be
[tex]\frac{Dsin\theta_1}{dsin\theta_5} = \frac{\lambda}{5\lambda}[/tex]
[tex]\frac{D}{d} = \frac{1}{5}[/tex]
[tex]d = 5D[/tex]
Therefore the slit separation is five times the slit width d=5D