Answer:
125
Explanation:
We're looking for a number that we can multiply three of the same number to get. For example, 25 is a perfect square, because 5*5 = 25.
You can test these all on your calculator, if you're allowed! Just take the third root of all of them, and if it's a whole number, you know you have a perfect cube.
The answer, though, is 125, because 5*5*5 = 125.