What number is a common denominator for 3/8and1/3

A common denominator is when the denominators (or the value in the bottom of the fraction) is made the same through multiplication, etc. 

24 is a common denominator for this example:

3/8(3/3) = 9/24
1/3(8/8) = 8/24

9/24 and 8/24 have common denominators, while their numerators are also multiplied, for 'what you do to the bottom, you do to the top' (sorry forgot the exact term for it)

hope this helps

Ais the answer for your question