Question

The Rockwell hardness of a metal is determined by impressing a hardened point into the surface of the metal and then measuring the depth of penetration of the point. Suppose the Rockwell hardness of a particular alloy is normally distributed with mean 69 and standard deviation 3.

(a) If a specimen is acceptable only if its hardness is between
68 and 76, what is the probability that a randomly chosen specimen
has an acceptable hardness? (Round your answer to four decimal
places.)

(b) If the acceptable range of hardness is (69 − *c*, 69 +
*c*), for what value of *c* would 95% of all
specimens have acceptable hardness? (Round your answer to two
decimal places.)

(c) If the acceptable range is as in part (a) and the hardness of
each of ten randomly selected specimens is independently
determined, what is the expected number of acceptable specimens
among the ten? (Round your answer to two decimal places.)

specimens

(d) What is the probability that at most eight of ten independently
selected specimens have a hardness of less than 71.52?
[*Hint*: *Y* = the number among the ten specimens
with hardness less than 71.52 is a binomial variable; what is
*p*?] (Round your answer to four decimal places.)

Answer #1

a)

for normal distribution z score =(X-μ)/σx | |

here mean= μ= | 69 |

std deviation =σ= | 3 |

probability that a randomly chosen specimen has an acceptable hardness :

probability =P(68<X<76)=P((68-69)/3)<Z<(76-69)/3)=P(-0.33<Z<2.33)=0.9901-0.3707=0.6194 |

b)

for middle 95% values, crtiical z =1.96

therefore c=1.96*3 =5.88

c)

expected number of acceptable specimens among the ten
=np=10*.6194 =**6.19**

d)

probability =P(X<71.52)=(Z<(71.52-69)/3)=P(Z<0.84)=0.7995 |

here this is binomial with parameter n=10 and
p=0.7995 |

P(X<=8)= |
∑_{x=0}^{a }
(_{n}C_{x})p^{x}(1−p)^{(n-x) }
= |
0.6257 |

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