are the following sentences correct?the price of these jeans is reasonable.

the quality of the candle is poor.
bread and butter are our daily food.
the books borrowed from the library are on my desk.
the famous singer and composer have arrived.
the boy who won the medals is a friend of mine.
collecting matchboxes is one of his favourite pastimes.
neither his father nor his mother play mahjong.
neither parent is fond of playing mahjong.
none of my friends were there

They are all correct, except for capitalization and punctuation (missing periods at the ends of sentences). I assume you are in Britain or its Commonwealth, since "favourite" is not spelled with a "u" in the USA.

These two have subject-verb agreement problems:

neither his father nor his mother play mahjong.

none of my friends were there

http://grammar.ccc.commnet.edu/grammar/sv_agr.htm

Let us know how you decide to correct them. Correct capitalization and punctuation are needed, too.

the famous singer and composer have arrived.

This sentence is correct if the singer and the composer are two different people. But if the singer is also the composer, it's incorrect.

I agree with Writeacher, and would have made the same mistakes myself. The words "neither" and "none" require singular third person verb conjugations.

Yes, all the sentences you provided are grammatically correct.