a 11 or an 11?

An eleven-to-twelve...

if we have to retain the the ratio term 11:12 can we use "an 11:12 ratio"?

Yes; that is what I said (or meant to say).

Anonymous our teacher explained it openly but not to face same problem again look up the pronounciation of the word.

For example university.Which one will you use a or an? Though it may seem you an because the word starts with a vowel it is spelled as (I can't write one of the symbols exactly)/'ju:ni'v3:siti/ Here j means you will spell it as you spell the vowel

DollAnonymous our teacher explained it openly but not to face same problem again look up the pronounciation of the word.

Typo: pronunciation

"A" or "an" is used before nouns according to their pronunciation, not their spelling.

Or before an adjective.

Or before an adverb.

(Is that an excessively minor piece of information?)

Oh, yes-- adverbs are much more minor than adjectives.