First of all, it's been a while since my last post -- I'm back in college and super busy. I'm doing a take home test now in statistics and I'm completely stuck on one problem... hoping anyone can help, please Please Register or Log in to view the hidden image! I've finished 24 of the 25 questions, double checked them all and I am sure that I have them all correct. However this question (#25) is frustrating me to no end -- and I'm coming to you with humility and shame. I just don't understand what I am doing wrong. I'm not simply asking for an answer here; I truly want to understand what I'm screwing up, so I'll post my method and then you can point out my error. Here goes... The question: A company installs 5000 light bulbs, each with an average life of 500 hours, standard deviation of 100 hours, and distrubution approximated by a normal bell curve. Find the % of bulbs that can be expected to last between 540 hours and 780 hours. My [wrong] answer: First I had to obtains a z-score for each piece of data. To do this I subtracted the data from the mean and divided my answer by the standard deviation. The z-score for 540 = (540-500)/100 = .4 The z-score for 780 = (780-500)/100 = 2.8 Then I took those z-scores and figured out their respective areas on the curve [using a nifty chart in my textbook]. The Area of z-score .4 = .016 The Area of z-score 2.8 = .497 Finally, I subtracted The area .016 from .497 So: .497 - .016 = .481 Multiply by 100 and the answer should be 48.1%, correct? No... My choices (it's a multiple choice exam) are 44.3%, 34.2%, 34.34%, and 44.34% !!! My answer does not match. I considered subtracting 48.1% for 50% since both scores are on the right side of the mean on the curve, but that didn't work either ---------------- Please help me. I am going crazy over this. I usually wouldn't ask for help on schoolwork, because I know this is not a "I'll do your schoolwork for you" sort of board, but I'm not asking anyone to simply give me an answer. I really want to understand my mistake and what I am doing wrong. Thanks.

Ok so far. Whoa! How do you get that? These answers don't make any sense. Think abut it this way: - 50% of the population lies below the mean, so certainly more than 50% of the population lies below the mean plus 0.4 standard deviations. - 2.8 is nearly 3, and 99.87% of the population lies below the mean plus 3 standard deviations. One of those choices is indeed correct.