A manufacturer of industrial solvent guarantees its customers that each drum of

Question

A manufacturer of industrial solvent guarantees its customers that each drum of solvent they ship out contains at least 100 lbs of solvent. Suppose the amount of solvent in each drum is normally distributed with a mean of 101.3 pounds and a standard deviation of 3.68 pounds.

asked this question previously, i thought it was right. it wasn't. previous answers a).6368 b).6878

please help
a) What is the probability that a drum meets the guarantee? Give your answer to four decimal places.  

b) What would the standard deviation need to be so that the probability a drum meets the guarantee is 0.97? Give your answer to three decimal places.

Explanation / Answer

Normal Distribution
Mean ( u ) =101.3
Standard Deviation ( sd )=3.68
Normal Distribution = Z= X- u / sd ~ N(0,1)                  
P(X < 100) = (100-101.3)/3.68
= -1.3/3.68= -0.3533
= P ( Z <-0.3533) From Standard Normal Table
= 0.3619                  
P(X > = 100) = (1 - P(X < 100)
= 1 - 0.3619 = 0.6381
b)
P ( Z < x ) = 0.97
Value of z to the cumulative probability of 0.97 from normal table is -1.881
P( x-u/s.d < 100 - 101.3/s.d ) = 0.97
That is, ( 100 - 101.3/s.d ) = -1.88
--> s.d = ( 100 - 101.3 ) / -1.88   = 0.6914          

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