I don't know, where the author of this example learned math, but (100 * 1.0) + (80 * 0.5) + (60 * (0.5 * 0.5)) is 155, not 105... (100 * 1.0) + (80 * 0.5) + (60 * (0.5 * 0.5)) = 100 + 40 + (60 * 0.25) = 140 + 15 = 155
