2^(n+4) - 2^n
=2^n * 2^4 - 2^n
=2^n (2^4-1)
=2^n(16-1)
=15 *2^n
=3 * 5 * 2^n
∴{ 2^(n+4) - 2^n } ÷ 3 = { 3 * 5 * 2^n } ÷ 3 = 5 * 2^n