Math Is Fun Forum

manich44

Member

Registered: 2009-11-29

Posts: 14

Probability I Think!!

Can someone explain how I go about the following...

Q. a large number of bricks were found to have a MEAN compresive 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 stength 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

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Probability I Think!!

Hi manich;

Using the chart here:

http://en.wikipedia.org/wiki/File:Normal_distribution_and_scales.gif

This is only a 2 digit after the decimal point chart. If you are using a chart with more accuracy then please adjust for small errors.

1) Needs slightly more clarification for an answer. We expect 68.26% of the bricks to be +1 and -1 standard dev. from the mean. 95.45 to be between -2 and 2 standard deviations from the mean.

2) 37.62 is +1 standard deviations from the mean, so the prob. that the compressive strength is less than that is ( .5 + .3413) = 84.13%

3) 33.48 is -2 standard deviations from the mean. The probability that a brick is stronger than that is the Area of the SNC from  -2 to inifinity.

13.59 + 34.13 + 50 = 97.72%

4) 34.13 + 34.13 + 13.59 = 81.85% with better charts ( mine are 6 digits after the decimal ) it is closer to 81.86%

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