Math Is Fun Forum

jack1234

Member

Registered: 2007-12-01

Posts: 2

Math/Stats help with Prob Distribution

having some problems figuring out this;
A used car dealership has found that the length of time before a major repair is required on the cars it sells is normally distributed with a mean equal to 10 months and a standard deviation of 3 months. If the dealer only wants 5% of the cars to fail before the end of the guarantee period, for how many months should the cars be guaranteed?
Any help would be greatly appreciated.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Math/Stats help with Prob Distribution

You'll need to look at a table of values (which I'm guessing you have, if you've been given this question). They tell you the probability that a random variable is less than Z standard deviations above the mean, for lots of values of Z.

So you need to use that table to find what Z-value gives a probability of 0.95, and then work out how many months is that many standard deviations away from the mean.

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Why did the vector cross the road?
It wanted to be normal.