For which value of n is n(n+1)/2 divisible by 4?
n ≡ 0 (mod 8)
n ≡ 3 (mod 4)
n ≡ 0 or 3 (mod 8) or n ≡ 7 or 4 (mod 8)
n ≡ 0 or 7 (mod 8)