The answer is 1/5.
First, You have to figure out if there are any numbers that go into both the numerator and denominator evenly.
This number, in this problem, would be three.
This is because 3/3=1 and 15/3=5.
Given that it asks for a fraction in the simplest form, we of course have order those numbers as numerator and denominator.
The three is the numerator, or, above the fifteen, or the denominator.
This must mean that the one is the numerator.
Because the 1 is the numerator, that must mean the 5 is the denominator, which brings us to the answer.... 1/5