Aim High's answer is correct, theory is almost right. Sorry to be a pedant!
Chart convergence represents the change in bearing of a straight line drawn on that chart. Earth convergence gives change of great-circle bearing. The question gives the straight line bearings, from which we can work out the change, 12°. This is itself, therefore, chart convergence. The proximity of the straight line to a great circle is not required.
The formula AH used was for chart convergence, so the exact change of longitude is 15° as correctly calculated.