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# poisson Watch

1. weak spots occur at random in the manufacture of a certain cable at an average rate of 1 per 100m. If x represents the number of weak spots in 100m of cable, write down the distribution of X.

Lengths of this cable are wound on to drums. Each drum carries 50m of cable. Find the probability that a drum will have 3 or more weak spots.

A contractor buys five such drums. Find the probability that 2 have just 1 weak spot each and the other three have none.

I know X~Poisson (1)
P(x is more than or equal to 3)
but since it's 50m cable, do I need to change lambda?
2. Your lambda has been given for 100m and you want to work out for 50m, so yes, you do need to change lambda. How do you change it?
3. (Original post by claret_n_blue)
Your lambda has been given for 100m and you want to work out for 50m, so yes, you do need to change lambda. How do you change it?
is lambda 1/2?
4. (Original post by 0utdoorz)
is lambda 1/2?
Yes.
5. (Original post by claret_n_blue)
Yes.
but when I do 1/2 lambda, the formula doesn't work and it's not in the tables?
6. oops i thought this was french, (poisson=fish)
7. (Original post by 0utdoorz)
but when I do 1/2 lambda, the formula doesn't work and it's not in the tables?
The formula should work, you have:

8. (Original post by 0utdoorz)
but when I do 1/2 lambda, the formula doesn't work and it's not in the tables?
Also, make sure you are using the right signs and values:

P( X >= 3 ) = 1 - P( X < 3 ) = 1 - [P(X = 0) + P(X = 1) + P(X = 2)]

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Updated: April 16, 2013
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