The problem is that English is not mathematically complete.... Please forgive me for duplication of info, but I didn't read all comments 'cause I'm lazy (except with apostrophes). What you are looking for is the word "Whicheth" which does not exist. "Whicheth prime minister of India is Manmohan Singh?" is the only question which is guaranteed to yeild the desired response, given that the recipient of your importune is aware of its intended meaning. (Correction apparently somebody has coined the term how-manyth, but I'm biased to whicheth, 'cause I've been using it for years) The problem with "Which prime minister..." is that I could always respond "The one with the grey hair," or some such nonsense, and still answer correctly.