[header] How to mentally calculate primality of any three digit number [title] Get pencil and paper. [step] Like any mathematical principle, you will remember it better if you practice on paper at the same time. [title] Estimate the square root of the number. 
[step] This will make your check faster since one only needs to try to divide the possible prime by the prime numbers less than the square root of that number. A quick reference might help: the square root of 100 is 10, of 225 is 15, of 400 is 20, of 625 is 25, and of 900 is 30.