# Math Help - Probability questions

1. ## Probability questions

Ok, I completely useles at probability and stats and don't understand these questions.

Computer parts are exponentially distributed and have a mean lifetime of 1000 hours, and are independent of each other.

What proportion are in excess of 100 hours?

Boxed in packages of 10, each box is to have at least 9 in excess of 100 hours. Calculate the percentage?

Also gold coins are found 1 per 1km sq. Jim has a metal detector and searches 10km sq.

Let N be the number of coins in that area. State the distribution X.

And the probability of finding a coin is 0.6. Let X be the number of coins found and N=n, what is the distribution of X. The calculate the mean of this distribution.

Is it a Poisson distribution?

2. Originally Posted by Happy Dancer
Ok, I completely useles at probability and stats and don't understand these questions.

Computer parts are exponentially distributed and have a mean lifetime of 1000 hours, and are independent of each other.

What proportion are in excess of 100 hours?

Boxed in packages of 10, each box is to have at least 9 in excess of 100 hours. Calculate the percentage?

[snip]
The pdf of X is $f(x) = 0.001 e^{-0.001 x}$.

(a) $\Pr(X > 1100) = \int_{1100}^{+\infty} 0.001 e^{-0.001 x}$.

(b) Makes no sense to me. Is that exactly how the question was asked?

Originally Posted by Happy Dancer

[snip]
Also gold coins are found 1 per 1km sq. Jim has a metal detector and searches 10km sq.

Let N be the number of coins in that area. State the distribution X. Mr F says: Do you mean N, not X here?

And the probability of finding a coin is 0.6. Mr F says: Finding a coin where? In 1 km sq? 10 km sq?

Let X be the number of coins found and N=n, what is the distribution of X. The calculate the mean of this distribution.

Is it a Poisson distribution?
I'm sorry but I cannot make sense of most of these questions the way you have posted them. You will need to post them more carefully and exactly as they are written in the book (or wherever it is you got them from) if you want to get help with them.