Plural or single noun with multiple adjectives

 cf: https://english.stackexchange.com/questions/221331/plural-or-singular-noun-when-it-refers-to-two-things-in-a-list

The followings are right.

I have a left and a right arm.

I have left and right arms.

Everyone has left and right arms. [This is correct]

The followings are wrong.

I have an old and a new cars.

I have old and new car.