Hello, This is my first post. Anyways, from the beginning, since I started learning the subjects at higher level, I have faced this problem - How to determine if the nos. is a prime no. ? The numbers under 100 are known to me, but if a bigger digit comes, are there any tricks to determining whether it is a prime no. or not? In one of my previous exams, I had found myself in trouble with just this small no. 151 ... and yeah, I found that it was prime within minutes, but I always try to divide the number by 7, 13, 17, and 19 to determine whether it is a prime or not. But this is just trial and error, so, could anybody, any mathgenius disclose me the trick? Require it badly for the upcoming exams - 1 day to go! Currently I am in XIth Grade.