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# S2 Edexcel Book review exercise 1 q8C Watch

1. Hi - i was wondering if someone could help. I am working through the edexcel book and am stuck on the question above. The question is:
the probability of a bolt being faulty is 0.3. Find the probability that in a random sample of 20 bolts there are
a) exactly 2 faulty bolts - answer is 0.0279
b) more than 3 faulty bolts - answer is 0.8929

these bolts are sold in bags if 20. John buys 10 bags.
c) find the probability that exactly 6 of these bags contain more than 3 faulty bolts.

so i have: X~B(10,0.8929) P(X=6) (10/6)(0.8929^6)(0.1071^4) but am getting the wrong answer. What am i doing wrong?

thanks
2. (Original post by Catful1)
Hi - i was wondering if someone could help. I am working through the edexcel book and am stuck on the question above. The question is:
the probability of a bolt being faulty is 0.3. Find the probability that in a random sample of 20 bolts there are
a) exactly 2 faulty bolts - answer is 0.0279
b) more than 3 faulty bolts - answer is 0.8929

these bolts are sold in bags if 20. John buys 10 bags.
c) find the probability that exactly 6 of these bags contain more than 3 faulty bolts.

so i have: X~B(10,0.8929) P(X=6) (10/6)(0.8929^6)(0.1071^4) but am getting the wrong answer. What am i doing wrong?

thanks
Hi there I did this question just a day ago and I'll be able to help First of all your binomial parameters are completely correct, remember you are missing one significant thing here, your factorials should be in place of 10/6 remember for binomial distribution
3. That first bit of the binomial formula is

That means , not .

4. Perfect! Thank you!

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