S2 question has me stumpedWatch
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!?