11 is only divisible by 1 and 11, it is a prime number. 21 is visible by 1, 3, 7, and 21; not a prime number. 51 is divisble by 1, 3, 7, 17, and 51; not a prime number. 11 is the only prime number .
Well, 25 certainly doesn't divide evenly into 595, but it does divide into 600, which is <em>very</em> close to 595. So if we divide 600 by 25, we can get a rough estimate of what 595 divided by 25 is.