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May 26th, 2010, 03:25 AM  #1 
Member Joined: May 2009 Posts: 90 Thanks: 0  Probability question
Can someone explain how I go about the following... Q. a large number of bricks were found to have a MEAN compressive strength of 36.24N/mm2, with a standard deviation of 1.38 N/mm2. 1. I need to find within what limits the compressive strength of the samples could be expected to lie. 2. the probability that a random brick would have a compressive strength less than 37.62N/mm2 3. the probability that a random brick would have a compressive strength more than 33.48N/mm2 4. the probability that a random brick would have a compressive strength between 34.86 N/mm2 and 39.00N/mm2 Thanks....Mark 
May 26th, 2010, 04:36 AM  #2 
Senior Member Joined: Apr 2008 Posts: 435 Thanks: 0  Re: Probability question
So there are a few methods to go about this, and I'm not really certain which you have learnt. But here are a few things you should look out for  when asked to describe where you should expect values to lie, this means that you give an interval and how confident you are that a random brick might lie in that interval. A common one is a 90% confidence interval. I might also expect that you normalize everything so that you can get an approximate normal distribution  but I don't know for certain whether or not this is what you really do. Presuming it is, you might have a table with values, or you might be expected to use some sort of calculator and formula. Really, you probably have the best idea of how you are expected to do this. What do you think? 
May 26th, 2010, 06:40 AM  #3 
Global Moderator Joined: Dec 2006 Posts: 20,968 Thanks: 2217 
A reasonable first step would be to state that you will assume, for calculation purposes, that the compressive strength of the bricks is normally distributed. The question appears to be designed to allow it to be answered without using a calculator, provided that you happen to know a few facts about the normal distribution. On that basis, are you able to make a start, or at least summarize what facts you know? 
May 26th, 2010, 07:28 AM  #4 
Global Moderator Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4  Re: Probability question
2. Does the fact that 36.24N/mm2 + 1.38 N/mm2 = 37.62N/mm2 not give you any hints? "689599.7 rule"?!?!? 

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