Wristwatch Exchange

Please help me with a homework problem of statistics?

I have an exam tomorrow and need help! if Please help, I panicked! the average lifespan of a wristwatch is 25 months with a standard deviation of 5 months. If the distribution is normal, how many months should be guaranteed if the manufacturer does not want to exchange more than 10% of the watches? Suppose that the variable is normally distributed. Response is 18.6 months …. How is it worked?

You need to look in the tables of normal distribution, the amount that leaves 90% outside. (I think is from -1.28 http://www.maths.leeds.ac.uk/ sta6ajb/math1725/tables.pdf ~) But this picture is where the mean = 0 and variance = 1 in our table , the critical value is the average – 1.28 sqrt (variance) = 25 to 1.28 x 5 25 = – 6.4 = 18.6 I've always found the easiest way to understand is to outline the bell curve / normal distribution with standard values of the mean (0) and standard deviation in pencil – almost the entire curve between 0 + / – 4 and then immediately below, write the corresponding values for the given curve – the average 25 will be written as 0, the value 25 5 (mean plus one standard deviateion) is immediately below 1, 25 + 2 times 5 is immediately below the normal 2 standard. It should help you understand the meaning. For this curve near 100% between 25 + 4 times the standard deviation (5) or 45 and the lower end of 25-4 after 5 or 5 months.

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