Thread: probability question

I am studying for a test, and came across this question wondering if anyone can help me solve this.

Neverready batteries lifetime is normally distributed. For a new type introduced for CD players, it was observed that the mean lifetie was 10.5 hours, and 90% lasted longer than 9 hours. If you take a random sample of 4 batteries, what is the probability that exactly 2 lasted longer than 11 hours?

thank you very much.

Originally Posted by shellshock

I am studying for a test, and came across this question wondering if anyone can help me solve this.

Neverready batteries lifetime is normally distributed. For a new type introduced for CD players, it was observed that the mean lifetie was 10.5 hours, and 90% lasted longer than 9 hours. If you take a random sample of 4 batteries, what is the probability that exactly 2 lasted longer than 11 hours?

thank you very much.

First find the variance of X (the lifetime of a battery) using the given information.

Then calculate p = Pr(X > 11).

If Y is the random variable 'number of batteries that last longer than 11 hours' then Y ~ Binomial(n = 4, p = as above).

Calculate Pr(Y = 2).

Originally Posted by mr fantastic

First find the variance of X (the lifetime of a battery) using the given information.

Then calculate p = Pr(X > 11).

If Y is the random variable 'number of batteries that last longer than 11 hours' then Y ~ Binomial(n = 4, p = as above).

Calculate Pr(Y = 2).

I'm sorry , can you show me how to find the variance?

Originally Posted by shellshock

I'm sorry , can you show me how to find the variance?

Read this thread: http://www.mathhelpforum.com/math-he...lp-please.html