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'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...)

It would have been impressive if I'd actually got all the way to the answer without using trial and error!

This is the official answer in the Puzzle Pad:-
The easiest approach is to see that if he works No days he will lose $180.
For every day he works, he gains $15 on this, being the $9 he works and the $6 which he doesn't forfeit.
So he must have worked 180/15 = 12 days.

Today's Puzzle :-
Can you complete the sequence of numbers, replacing the question mark?

1, 2, 5, 10, 20, ?

50

I agree 50

If the answer is 50, what would be the next number?

100

^^^ wot he said, and the next would be 200