words - How do we decide if 中 is ちゅう or じゅう?

How do we decide if 中 is ちゅう or じゅう ?

For example, in this sentence １０人中３人はビデオを持っている which pronunciation should we use?

What about this: そのピアニストの演奏中、彼らは一心に耳を傾けた。

Answer

• じゅう (or, rarely: ぢゅう) is used for expressions covering a length of time (resp. area) from start to finish, in its entirety. I.e. "through", "all of":

一年中【いちねんじゅう】 all year

一日中【いちにちじゅう】 all day

世界中【せかいじゅう】 all over the world

• ちゅう is for pointing a particular time (resp. specific location) out of an interval (resp. general area). I.e. "out of", "during":

午前中【ごぜんちゅう】 [at some point] during morning

会議中【かいぎちゅう】 in a meeting [e.g. "he is in a meeting at the moment"]

Note that in some cases, both can be used and the only way to tell would be from context (e.g. an expression followed by に is more likely to be ちゅう) or in speech:

来月中【らいげつじゅう】 all of next month

来月中【らいげつちゅう】 [some time] next month