Simplifying fractions are easy. You find a number that will go evenly into the top and bottom of the fraction.
If you cannot find a number that will that, it means the fraction is it's simplest form.
Now, what will go into 15 and 64 evenly?
I first thought of 3. While that will go evenly into 15, it won't for 64.
I then thought of 5 but of course that won't do either.
I then realized this fraction is already in it's simplest form! So, you can't reduce this fraction anymore.