HINT 20A: Fitness is proportional to W = (7/31)v_A + (24/31)v_B. Draw contours of fitness: For example, W = 1 = (7/31)v_A + (24/31)v_B is a downward-sloping straight line. The optimal survival maximizes fitness, given the constraint v_A² + v_B² ≤ 1, and is where fitness contours just touch the trade-off curve in Figure P20.1.