If you are asking what I think than it should be 5/5 because 3*5=15 so in that case the denominator would be 15 making the whole answer 10/15 although that could still be simplified to 2/3) (that's what happens when you multiply by one)
Or you may be asking 1/x forms a denominator of 15. In this case you would still have 5 as your number (or really 1/5), if would just mean when they are multiplied together you would get 2/15 instead....
