n is divisible by 3?
Whenever a number is divided by 3, the remainder obtained is either 0 or 1 or 2. ∴ n=3p or 3p+1 or 3p+2, where p is some integer. If n=3p, then n is divisible by 3. If n=3p+1, then n–1=3p+1–1=3p is divisible by 3.