Standard +0.3 This is a straightforward proof by induction with a divisibility statement. The base case is trivial (n=1 gives 77-13-1=63, divisible by 3), and the inductive step requires standard algebraic manipulation to factor out 3 from the difference. While it's a Further Maths question, it follows the standard induction template without requiring any novel insight or particularly complex algebra, making it slightly easier than average.