Re: English question

To my horror, I have found something in English I do not know how to
punctuate.

An apostrophe is used in plurals after numbers and letters, so we are right
to say "the 30's", and "6 CD's", or "6 GP's".

What do we do if we wish to indicate possession?
"My GP's surgery" is no problem, but what about "6 GP's' opinion"?

Two apostrophes can't be right, but one isn't right in either position.
Any ideas? Or anyone got a copy of Fowler?

Peter