S2 question has me stumped

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Thread starter 15 years ago
#1
OK here we go...i dont know why i'm getting this wrong:

Weak spots occur in a cable on average once per 100 metres.

Drums of cable contain 50 metres worth of cable.

Find the probability that a contractor buying 5 drums finds that two drums have exactly 1 fault each, and the other 3 have no faults.

So i did this:

Poisson distribution with mean = 50 * 1/100 = 0.5 mean fault per drum

so P(X=1) = e^-0.5 * 0.5 = 0.303

if Y is number of drus with a single fault, then Y is a binomial distribution:

P(Y=2) = 5c2(0.303)^2(1-0.303)^3 = 0.311

But the answers in the book says 0.205

sometimes the books answer is wrong...is that the case here or is my method wrong? If so, why!?
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Thread starter 15 years ago
#2
ignore me - i solved my own question - probability of no faults is not 1-0.303
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