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

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28.2 Introducing probability 907

Engineeringapplication28.2

Qualitycontrolonafactoryproductionline

Quality control is an important aspect of factory production. So much so that some

engineersspendtheirwholecareersinthisfieldandareknownasqualitycontrolengineers.Thefundamentalgoalistoensurethatmanufacturedgoodsareproducedto

aspecifiedstandardinspiteofvariationsinthefinishedproductthatareaninevitable

partof the manufacturing process.Consider the following problem.

Machines A and B make components, which are then placed on a conveyor belt.

OfthosemadebymachineA,93%areacceptable.OfthosemadebymachineB,95%

areacceptable.MachineAmakes60%ofthecomponentsandmachineBmakesthe

rest. Find the probability that a component selected at random from the conveyor

belt is

(a) made by machine A

(b) made by machine Aand acceptable

(c) made by machine Band acceptable

(d) made by machine Band unacceptable

Solution

(a) We are given that 60% of the components are made by machine A. Converting

thispercentage toadecimal number gives

P(component ismade by machine A) = 0.6

(b) Weknowthat60%ofthecomponentsaremadebymachineAand93%ofthese

areacceptable. Converting these percentages todecimal numbers wehave

P(component ismade by machine Aand is acceptable) = 60

100 × 93

100

= 0.60 ×0.93 = 0.558

(c) Weknowthat40%ofthecomponentsaremadebymachineBand95%ofthese

areacceptable. So,

P(component ismade by machine Band isacceptable) = 40

100 × 95

100

= 0.40 ×0.95 = 0.38

(d) We know that 40% of the components are made by machine B and 5% of these

areunacceptable. So,

P(component ismade by machine Band isunacceptable) = 40

100 × 5

100

= 0.40 ×0.05 = 0.02

An alternative way to solve this problem is by constructing a tree diagram. To

do so, consider the case when 1000 components are picked from the conveyor

belt. We know that machine A makes 60% of the components and so 600 of these

components, on average, will be made by machine A and the other 400 must

therefore be made by machine B. This isshown inFigure 28.3.

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