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# S2 Help-Binomial and Poisson Watch

1. Hi guys, can u help me wit this question:
Flaws in a certain brand of tape occur at random and a rate of 0.75 per 100 m
Assumin a Poisson is used for the no of flaws in a 400 m roll of tape.
a) Find the probability of at least one flaw (Done)
b) Shows that probability that at most ther will be 2 flaws is 0.4232 (Done)

In a batch of 5 rolls, each of length 400m
c) Find the probability that 2 rolls will contain less than 5 flaws (Need help )

thanks
2. You still need help with this mate??

These are the type of question that you can easily get stuck on in an exam because you are expecting to use your knowledge of poisson but it isn't the case with (c)

You can see that the probability of any given roll of tape having less than 5 flaws is .8153

If your talking about 5 different rolls and you want the probability of two of them containing less than 5 flaws, then there are 10 different ways this can happen (5 C 2 )

SO this gives us a binomial distribution, X - (5 , .8153)

You want P(X=2) --> 5C2 x (8153)^2 x (.1847)^3

this should give you the answer, quote me and let me know
3. (Original post by MikeySwansea)
You still need help with this mate??

These are the type of question that you can easily get stuck on in an exam because you are expecting to use your knowledge of poisson but it isn't the case with (c)

You can see that the probability of any given roll of tape having less than 5 flaws is .8153

If your talking about 5 different rolls and you want the probability of two of them containing less than 5 flaws, then there are 10 different ways this can happen (5 C 2 )

SO this gives us a binomial distribution, X - (5 , .8153)

You want P(X=2) --> 5C2 x (8153)^2 x (.1847)^3

this should give you the answer, quote me and let me know
Thats what i was thinkin for part (c) as well, but yur answer gives a probabilty of 0.04188, but the book says 0.702
Btw thanks for yur help
4. That answer seems way too high for me.

Would love to see what others think as I could easily be wrong.
5. (Original post by MikeySwansea)
That answer seems way too high for me.

Would love to see what others think as I could easily be wrong.
The OP's wording of the question is incorrect, see:

6. Cheers for that Ghostwalker.
7. Oh thnx for that guys, appreciate it
8. ooh guys, is ther a quicker way to calculate the binomial or poisson distribution on the calculator if the parameter is not in the table and the formula is too long to use
9. (Original post by sl96)
ooh guys, is ther a quicker way to calculate the binomial or poisson distribution on the calculator if the parameter is not in the table and the formula is too long to use
Not that I'm aware of, and I would think it unlikely you'd get anything requiring a great deal of computation at A-level.

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