A lot of civilian firms now include "Numeracy assessment" in their selection process (just been there, just done that!), so there are handy booklets available in the "Business" section of your local W~t*rst#n*s or whoever.
They seem to have catchy titles like "How to do well in Numeracy Assessment", and the sums are business rather than aviation, but basic principles are the same; you may find them useful.