125 = 5^3 = 25 * 5
The simplification you are trying to do involves finding the prime factorization of a number. Since 125 is not divisible by 2, move on to 3. 125 is also not divisible by 3, so move on to 5. Then 125/5 = 25, and 25/5 = 5, so 125 = 5^3. The prime factorization of 125 is 5^3.