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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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912 Chapter 28 Probability

(c) P(component lasts less than 4 years) =P(C ∪D)

=P(C) +P(D)

=0.37 +0.08

=0.45

Engineeringapplication28.4

Calculatingtheprobabilityacomponentisacceptableon

afactoryproductionlinewithseveralmachines

Sometimesafactoryproductionlinemaybefedbyseveralmachinesthatmakecomponents.

It is important to be able to calculate the overall quality of the product that

emerges given knowledge of the performance of the individual machines. Consider

the following problem.

MachinesAandBmakecomponents.MachineAmakes60%ofthecomponents.

The probability that a component is acceptable is 0.93 when made by machine A

and0.95whenmadebymachineB.Acomponentispickedatrandom.Calculatethe

probability thatitis

(a) made by machine Aand isacceptable

(b) made by machine Band isacceptable

(c) acceptable

Solution

WehavealreadylookedatthisprobleminEngineeringapplication28.2.Figure28.4

shows the tree diagram for the problem.

(a) P (component ismade by machine Aand isacceptable) = 558

1000 = 0.558.

(b) P (component ismade by machine Band isacceptable) = 380

1000 = 0.38.

(c) Note that the events described in (a) and (b) are mutually exclusive and so the

addition lawcan beapplied.

P(component is acceptable) =P(component ismade by machine Aand isacceptable)

+P(component ismade by

machine Band isacceptable)

= 0.558 +0.38

= 0.938

We can now obtain this probability directly from the tree diagram. We see that

558 +380 = 938 components areacceptable and so

P(component isacceptable) = 938

1000 = 0.938

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