Moderate -0.3 This is a straightforward proof by induction with a simple divisibility statement. The base case is trivial (7×1-15=-8, divisible by 4), and the inductive step requires only basic algebra to show 7×9^(k+1)-15 = 9(7×9^k-15)+120, both terms divisible by 4. While it tests understanding of induction structure, it requires no novel insight and is mechanically simpler than average A-level proof questions.