# Math - Standard Deviation

posted by
**GetYourLeash**
.

A General Electric soft white 3-way bulb has an average life of 1200 hours with a standard deviation of 50 hours. Find the probability that the life of one of these bulbs will be between 1150 and 1300 hours.

You need to change the times to Z-scores â€” scores given in terms of standard deviations.

Z = (X - µ)/SD, where X is the particular value, µ = the mean and SD is the standard deviation.

Once the Z scores are obtained, look it up in a table in the back of your statistics textbook called something like "areas under the normal distribution." Find the proportions between those Z-scores and the mean for both values and add them together to get the probability that the life of the bulb will be between these two values.

Although I did not solve the problem completely for you, I did tell you the process for reaching a solution. This will mean that you will have to exert a little more effort, time and thinking, but I hope it will help you to learn more.

I hope this helps. Thanks for asking.