question on probability... HELP!!!

[coyote]

I hope I get all the required info in this... I can't figure it out. PLease help!

In a call center, the people who answer the phones spend and average of 6.5 minutes on each call and 3.5 minutes between calls. If their supervisor walks by their desk 4 times, what is the probability that he will not find them talking on the phone on any of those times he passes by their desk?

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[zerohour]

The probability that the supervisor walks by the desk and finds the person not talking on the phone for one visit is 3.5/[3.5+6.5] = 3.5/10

The probability that he finds the person not taking calls in visis is [3.5/10]^4.


This is actually a binomial type problem and can be solved by binomial theoram also.