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.