When we subtract 1 or add 1 to a prime number. It is divisible by 6 or in other words Every Prime Number can be expressed as 6k±1. e.g. - 5=6×1-1 7=6×1+1 11=6×2-1 13=6×2+1 [ EXCEPTION=2 & 3 ] But I didn,t understand why this happened. Can anyone help me by telling the main reason behind this.