I am one and ninth when alone; two and eleven when doubled; four and five when am with the 21st; nine and eleven when I am with the 24th. What am I?
The letter I. (using the Roman figures and the English alphabet). When the letter "I" is alone, it is roman figure one (I) and the ninth letter of the English alphabet; when doubled, it is roman figure two (II) and 11. When it is written before and after the letter V (the 21st letter of the English alphabet), it becomes four and five respectively (IV & VI). So also with X (the 24th letter of the English alphabet), then it becomes nine & eleven (IX & XI) respectively.