#230, " C" is right answer , if we assume premise (1) the number of cleaners is between 10 and 21, exclusive. only " 15 cleaners * 14 rooms= 210" fits . But if premise turns out to be that the number of cleaners is between 10 and 21 inclusive" , then the answer will be " E" . B/c we should take two suitations into account: (1) 15 cleaners* 14 rooms , or (2) 21 cleaners *10 rooms.