# Homework Help: Exponential Distribution

1. Sep 23, 2004

### Changoo

I am having alot of trouble with a homework question from my book. It asks:

The time between unplanned shutdowns of a power plant has an exponential distribution with a mean of 20 days. Find the probability that the time between two unplanned shutdowns is more than 21 days.

I know this much so far 1-e-(20)(?) (one minus e to the negative power of mean times any value of the continuous variable(X))

I am lost on finding X within the equation.

Hope someone can help.

2. Sep 23, 2004

### courtrigrad

Hello!

First of all what do we define as an exponential distribution? It is equaled to:

1 - e^(-yx)

= 1 - e^(-20)(21)

Since this is the probability that the time between two unplanned shutdowns is less than 21 we don't need the 1. Hence our answer (I think) would be:

e^(-20)(21) or essentially 0.

Hope this helps!

Last edited: Sep 23, 2004
3. Sep 24, 2004

### Changoo

Thanks for your help,

But how do I solve E^-(20)(21)? I know that 20 times 21 is 420. How do I determine e^-420?

The book has given me some answers, but none say zero. a. .350, b. .650, c. .150, d. .850

I can probabily figure out the answer with no problem if someone can help me witht the problem above.
Thanks for your help!!!

4. Sep 24, 2004

### Changoo

Okay, here is what I have, please tell me if I am right:

F(x)=1-e^-(20)(2/21)

F(x)=1-.850

F(x)=.150 (final answer)

I hope I am write.

5. Sep 24, 2004

### Changoo

I meant right**** Sorry

6. Sep 24, 2004

### Changoo

Asking for Review

I feel confident about my answer, I am hoping someone can review and let me know if I have calculated wrong in any way.

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