Yes, all square roots of non square numbers are irrational. That means that the sqrt of a whole number will never be a fraction, but always an irrational number or an integer.
Second question, not entirely sure who "it" is...
The divisors of 2205 are: 1, 3, 5, 7, 9, 15, 21, 35, 45, 49, 63, 105, 147, 245, 315, 441, 735, 2205.
Trying them out small to large:
√(2205/3) =
27,11088... irrational
√(2205/5) = 21 rational!
So 5 is a divisor because 5*441 = 2205. Also √441=21 and 21 is rational.
