The Lifetimes of a certain brand of photographic light are normally distributed with the mean of 210 h and a standard deviation of 50 h.

Question

What percent of lights will need to be replaced within 233 h?

Out of 2000 lights, how many will have a lifetime between 200 h and 400 h?

Answer 1

Z = (score-mean)/SD

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportions related to the Z scores.