Problem regarding diffraction

In the case of single slit diffraction, is it necessary that the distance between slits must be less or equal than the wavelength of light in order to calculate angular width of dark fringes?

I got this question from my text book.It said:

if the distance between edges of a single slit diffraction is 0.1mm and the wavelength of light is 5500 *10^(-10)m. Determine the angular width of central maxima.If not possible,explain why?

The answer said it was not possible.

Please help me on this problem.