a) ∑(COMB(100, x)·(1/3)^x·(2/3)^{100 - x}, x, 31, 100) = 72.34%
b)
μ = n·p = 100·1/3 = 33.33
σ = √(n·p·(1 - p)) = √(100·1/3·2/3) = 4.714
Φ(k) = 0.9 --> k = 1.282
μ + 1.282·σ = 33.33 + 1.282·4.714 = 39.4
39 Essen sollten ausreichen
Probe
∑(COMB(100, x)·(1/3)^x·(2/3)^{100 - x}, x, 0, 39) = 90.34%