Central fringe physics !!

A narrow beam of light is directed normally at a slit.
a) blue filter placed in the path of a beam before it reaches the slit. distance across 5 fringes including the central fringe = 18mm find the width of the central fringe

The central fringe is double the width of the other fringes, so we can take the total width of 5 fringes including the central fringe to be equal to 6 "outside fringes"

18/6 = width of subsidiary fringes = 3mm

because central fringe = 2xsubsidary fringes
= 2*3 = 6mm

If you then add 3+3+6+3+3 (the widths of all the fringes in order, the answer = 18mm, so central fringe = 6mm

(I hope this makes sense, it did to me haha)