55% attendance: find prob. he attends >= 8 of 10 classes

Angela123

Junior Member
Joined
Oct 9, 2008
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54
Assume that he attends randomly with probability 0.55
and that each decision is independent of previous attendance, so that the
process can be viewed as a Bernoulli process.

What is the probability that he attends at least 8 of 10 classes
given that he attends at least 2 but not all 10 classes?

I thought it would be easier to calculate the probability for 0, 1, and 10 and then add those answers together and subtract it from 1.
I tried this:
(.55^0)*(.45^10)*C(10,0)=3.41*10^-4
(.55^1)*(.45^9)*C(10,1)=.0042
(.55^10)*(.45^0)*C(10,10)=.0025329516
I added those answers together and subtracted it from 1 and got .993- This seems right to me, but the homework says it's wrong
 
Re: Probability

Angela123 said:
Assume that he attends randomly with probability 0.55
and that each decision is independent of previous attendance, so that the
process can be viewed as a Bernoulli process.

What is the probability that he attends at least 8 of 10 classes
given that he attends at least 2 but not all 10 classes?

I thought it would be easier to calculate the probability for 0, 1, and 10 and then add those answers together and subtract it from 1.
I tried this:
(.55^0)*(.45^10)*C(10,0)=3.41*10^-4
(.55^1)*(.45^9)*C(10,1)=.0042
(.55^10)*(.45^0)*C(10,10)=.0025329516
I added those answers together and subtracted it from 1 and got .993- This seems right to me, but the homework says it's wrong

Where is the problem - who attends what where???
 
Re: Probability

Oops! Sorry I left out some details. Here is the real problem:

Assume that Bob attends class randomly with probability 0.55
and that each decision is independent of previous attendance, so that the
process can be viewed as a Bernoulli process.

What is the probability that he attends at least 8 of 10 classes
given that he attends at least 2 but not all 10 classes?
 
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