a)
P(kein stopp) = ∑(COMB(40, x)·0.08^x·0.92^{40 - x}, x, 0, 3) + ∑(COMB(40, x)·0.08^x·0.92^{40 - x}, x, 4, 6)·∑(COMB(20, x)·0.08^x·0.92^{20 - x}, x, 0, 2) = 0.8856766940
P(stopp) = 1 - P(kein stopp) = 1 - 0.8856766940 = 0.1143233060 = 11.43%
b)
P(X >= 2) = 1 - ∑(COMB(8, x)·0.08^x·0.92^{8 - x}, x, 0, 1) = 0.1297593020 = 12.98%