What is the common denominator for four over nine?

Answer 1

A single number can not have a "common denominator", because a "common denominator" is a denominator which can be used to express both numbers.

For example 1 / 2 and 1/3 have common denominators: 6, 12, 18, 24, 30, 36, etc... because you can express 1/2 as 3/6, 6/12, 9/18, 12/24, 15/30, 18/36 etc... and you can also express 1/3 as 2/6, 6/18, 8/24, 10/30, 12/36 etc...

A non example is 9, which is NOT a common denominator of 1/2 and 1/3, because while you can express 1/3 as 3/9, 1/2 with a denominator of 9 would be 4.5/9, which is not a proper fraction.