Answer:
D / d = 3 / 4
Explanation:
Let the wavelength and distance of screen be λ and L . Let the distance at which both the fringes fall be at x distance from central point .
For double slit experiment , location of fourth order maxima
x = 4  x λ L / d
For single  slit experiment , location of third  order minima
x =  3  x λ L / D
As they overlap
3  x λ L / D = 4  x λ L / d
D / d = 3 / 4