1. According to Chemical Engineering Progress (November 1990), approximately 30%
ID: 3244949 • Letter: 1
Question
1. According to Chemical Engineering Progress (November 1990), approximately 30% of all pipework failures in chemical plants are caused by operator error.
a) What is the probability that out of the next 20 pipework failures at least 10 are due to
operator error?
b) What is the probability that no more than 4 out of 20 such failures are due to operator
error?
c) Suppose, for a particular plant, that out of the random sample of 20 such failures, exactly
5 are due to operator error. Do you feel that the 30% figure stated above applies to this plant? Comment.
Explanation / Answer
a) P(X 10)
= 1P(X9)
= 10.9520
= 0.0480.
b) P(X 4) = 0.2375
c) P( X= 5) = 0.1789.
This probability is not very small so this is not a rare event.
Therefore,P= 0.30 is reasonable.
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