One use of the letter P is in probability, with P10, P50 and P90 describing the various levels of probability. Again, there is no space between the capital P and the number.
We do not use “p.” as an abbreviation for “page” in the text.
Example: See page 37.
Nor do we use it in the references when listing page numbers in a book or journal.
Example: Peaceman, D.W. 1990. Discussion of Productivity of a Horizontal Well. SPE Res Eng 5 (2): 252-253.
(Don Peaceman died recently. Rest in Peace, man.)
Now for the Qs.
Note that we do not use an apostrophe when we make the plural of a letter (i.e., not P’s and Q’s).
When we are describing which quarter of the year something will be done, we put the Q in front of the number, followed by a space and the four-digit year.
Example: Q4 2009, not 4Q09
So where did this expression “mind your Ps and Qs” come from, you ask?
There are several interesting theories, including not forgetting one’s pea jacket and queue wig when leaving the pub. However, a more likely provenance is a teacher reminding children learning to write not to mix up the lower-case letters p and q.