Why does single slit closer to double slit increase interference brightness?

[desmond iking]

Homework Statement

in my textbook, it states that if the single slit together with the light source more closer to the double slit, the intensity of light form double slit increases . hence, the bright fringes of the inteference pattern would be brighter...i couldn't understand why . Can someone explain on this?

Homework Equations

The Attempt at a Solution

 

[BvU]

Well, I don't understand either. Perhaps the wording got distorted. Could you please rad what you typed and correct the errors ? Otherwise: What textbook might that be ? Does it also provide a drawing of the situation ?