Intensity of single slit vs double slit

[Snydes]

Homework Statement

Light of intensity I0 passes through two sets of apparatus.
One contains one slit and the other two slits. The
slits have the same width. What is the ratio of the out
going intensity amplitude for the central peak for the
two-slit case compared to the single slit?

Homework Equations

The text doesn't give any, I assume it has to do with constructive and destructive interference from the light waves.

The Attempt at a Solution

Tried using equations for intensity that I've been given from other classes for double and single slit intensity but doesn't really leave me anywhere useful, plus if they're not given in the text then that's probably not the way to go about it. I've also considered superposition.
The answer turns out to be that the intensity of the double slit is larger by a factor of 4 but don't know how to get there

 

[BvU]

What does the tekst give ? You're invited to come up with some relevant equations, not with summing up what isn't given. Perhaps you want to search in earlier textbooks what you can find about single slits, double slits, amplitudes, intensities ?