Rule 11
The plurals for capital letters and numbers used as nouns are not formed with apostrophes.

Examples:
She consulted with three M.D.s.
BUT
She went to three M.D.s' offices.
The apostrophe is needed here to show plural possessive.
She learned her ABCs.
the 1990s not the 1990's
the '90s or the mid-'70s not the '90's or the mid-'70's
She learned her times tables for 6s and 7s.

Exception:
Use apostrophes with capital letters and numbers when the meaning would be unclear otherwise.

Examples:
Please dot your i's.
You don't mean is.
Ted couldn't distinguish between his 6's and 0's.
You need to use the apostrophe to indicate the plural of zero or it will look like the word Os. To be consistent within a sentence, you would also use the apostrophe to indicate the plural of 6's.

M

Rule 13
If the gerund has a pronoun in front of it, use the possessive form of that pronoun.

Examples:
I appreciate your inviting me to dinner. I appreciated his working with me to resolve the conflict.

Apostrophe rules part 4

Rule 12
Use the possessive case in front of a gerund (-ing word).

Examples:
Alex's skating was a joy to behold.
This does not stop Joan's inspecting of our facilities next Thursday.