# Thread: probability question

1. ## 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.

2. 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).

3. 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?

4. Originally Posted by shellshock
I'm sorry , can you show me how to find the variance?