its about a manufacturer performing a test on its calculators and finds their work life to be normally distributed. OK if I have a mean u=2150 hours and a standard deviation of 450 hours.
What should be advertised as the life of the calculators so that 90% of the calculators are covered ?
a.2,555 b.1947 c.1,410 d.1,745 and e.1,574 thanks

do not solve for me just show me the path, I missed the class and I just want to know the technic to solve it and I will be able to do the rest of my HW bms.

Step 1 - determine how many standard deviations from the mean do you need to be in order for 90% of the population to be to the right. Your text probably has a list of "z-scores" with this info. As an example, see: Distribution Tables (http://www.statsoft.com/textbook/sttable.html) These tables provide the cumulative probability between the mean and the given z-score, so you're actually looking for what z-score gives you 0.4 cumulative probability.

Step 2 - Multiply the z-score by the standard deviation for this population.

Step 3 - subtract the result of Step 2 from the mean.
Hope this helps.