Math Is Fun Forum

jubadedo

Member

Registered: 2015-04-05

Posts: 19

More Probability Help

Thanks guys, I really appreciated your help on the last questions. I need help though now. I don't understand how you approach and solve these problems...

An auto store has two suppliers for a given part. The store orders from Supplier A 70% of the time, and supplier B 30% of the time. Supplier A's parts are defective with probability 0.02, and supplier B's parts are defective with probability 0.05.

1. If I buy a part, what is the probability that it is defective?

2. I buy a part and it is defective. What is the probability that it comes from Supplier A?

3. Let's assume that the parts arrive from the factory in boxes of 10; if I pick two parts from the same box then I know they are from the same factory (but I don't know which factory they are from). If I buy two parts from the same box, what is the probability that they are both defective? Hint: What is the probability that two parts from supplier A are both defective? How about two parts from supplier B?

Nehushtan

Member

Registered: 2013-03-09

Posts: 957

Re: More Probability Help

I will give you a hint for #1 and #2 (you already have a hint for  #3). Given a batch of 1000 parts, 700 of them will be from Supplier A and 300 will be from Supplier B. Of the 700 from Supplier A, 14 will be defective, and of the 300 from Supplier B, 15 will be defective. So there will be 29 defective parts altogether. So...

#1. What's the probability of picking one of the 29 defective parts in a batch of 1000?

#2. Of the 29 defective parts, 14 are from Supplier A and 15 are from Supplier B. What is the probability of picking one of the 14 Supplier A parts from the defective bunch?

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Relentless

Member

Registered: 2015-12-15

Posts: 631

Re: More Probability Help

Hello!

1. It is like the other problem with ice creams. You need to multiply each probability by the percentage you pick that supplier

2. Once you get your two products from question one, you need to find out Supplier A's product as a percentage of the total

3. Two parts being defective is the probability of one being defective squared. I guess it is a bit like question one, except you are multiplying the probabilities twice

Nehushtan,

Sorry if I gave too much away

Last edited by Relentless (2016-02-15 06:04:21)

jubadedo

Member

Registered: 2015-04-05

Posts: 19

Re: More Probability Help

Okay so here's what I think are the answers, thanks again guys you're awesome for helping me out!

1. 0.7 * 0.02 + 0.3 * 0.05 = 0.029

2. .(0.7 * 0.02 + 0.3 * 0.05) /.7 I think

3. (0.7 * 0.02 + 0.3 * 0.05 ) * (0.7 * 0.02 + 0.3 * 0.05 ) = 0.000841

Last edited by jubadedo (2016-02-15 21:46:46)

Nehushtan

Member

Registered: 2013-03-09

Posts: 957

Re: More Probability Help

Only #1 is correct.

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Relentless

Member

Registered: 2015-12-15

Posts: 631

Re: More Probability Help

Hint ... question 1 answer is the denominator of question 2

Hint two ... The 0.02 and 0.05 bits are the probabilities I meant, not the whole thing

thickhead

Member

Registered: 2016-04-16

Posts: 1,086

Re: More Probability Help

For 2. use the formula
P(A/D)=  {P(A)*P(D/A)}/ {P(A)*P(D/A)+P(B)*P(D/B)}= {0.7*0.02} /{0.7*0.02+0.3*0.05}

Last edited by thickhead (2016-04-17 03:11:57)

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{1}Vasudhaiva Kutumakam.{The whole Universe is a family.}
(2)Yatra naaryasthu poojyanthe Ramanthe tatra Devataha
{Gods rejoice at those places where ladies are respected.}

thickhead

Member

Registered: 2016-04-16

Posts: 1,086

Re: More Probability Help

Last edited by thickhead (2016-04-17 03:23:59)

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{1}Vasudhaiva Kutumakam.{The whole Universe is a family.}
(2)Yatra naaryasthu poojyanthe Ramanthe tatra Devataha
{Gods rejoice at those places where ladies are respected.}