Puzzle a Day!

I've no idea what Mark or Pros's answers are but, this is my version.
Work day = £9
No work day = £6.
Therefore, 2 days worked (£18) = 3 days not worked.
Add 2 days + 3 days = 5
30 days in month divided by 5 = 6.
6 x2 days worked = 12
6 x 3 days not worked = 18.
12 + 18 days = 30.
Answer = 12.

I think you and Mark did it the same way, but there's a typo in Mark's working (he put 5 x 2 instead of the 6 x 2 he intended).

This seems a much less convoluted (and successful) method than my attempt to dredge up GCSE maths! (I did get an A, but it's a veeeeery long time ago...)