\"Durable press\" cotton fabrics are treated to improve their recovery from wrin

Question

"Durable press" cotton fabrics are treated to improve their recovery from wrinkles after washing. Unfortunately, the treatment also reduces the strength of the fabric. The breaking strength of untreated fabric is normally distributed with mean 50.7 pounds and standard deviation 2.7 pounds. The same type of fabric after treatment has normally distributed breaking strength with mean 25.8 pounds and standard deviation 1.7 pounds. A clothing manufacturer tests 3 specimens of each fabric. All 6 strength measurements are independent. (Round your answers to four decimal places.) (a) What is the probability that the mean breaking strength of the 3 untreated specimens exceeds 50 pounds? (b) What is the probability that the mean breaking strength of the 3 untreated specimens is at least 25 pounds greater than the mean strength of the 3 treated specimens?

Explanation / Answer

NORMAL DISTRIBUTION
the PDF of normal distribution is = 1/ * 2 * e ^ -(x-u)^2/ 2^2
standard normal distribution is a normal distribution with a,
mean of 0,
standard deviation of 1
equation of the normal curve is ( Z )= x - u / sd/sqrt(n) ~ N(0,1)
mean ( u ) = 50.7
standard Deviation ( sd )= 2.7
sample size (n) = 3
a.
P(X > 50)= (50-50.7)/2.7/ Sqrt ( 3 )
= -0.7/1.559= -0.4491
= P ( Z >-0.4491) From Standard Normal Table
= 0.6733
b.
P(X < 25) = (25-25.8)/1.7/ Sqrt ( 3 )
= -0.8/0.9815= -0.8151
= P ( Z <-0.8151) From Standard NOrmal Table
= 0.2075
P(X > = 25) = 1 - P(X < 25)
= 1 - 0.2075 = 0.7925

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