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So, the task is: There are two companies, A and B. The first company A uses the technology which consists of two operations in a row to get a product. The probability of getting a defect product from first operation is 0.4 and from the second one 0.5. The company B uses another technology which consists of three operations. The probability of getting a defect product from first operation is 0.1,from the second 0.2 and from the third 0.3. In company A there are 90% of first class products among working products (which are not defect) and in company B there are 70% of first class products among working products. Which technology (A's or B's) has a bigger probability of giving a first class product? This task sounds confusing to me and I'm not sure where to start...

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This is really simple. For company A, you want the probability that:

  • First step goes well ($P=1-0.4=0.6$) AND
  • Second step goes well ($P=1-0.5=0.5$) AND
  • You win the raffle? ($P=0.9$)

Whenever you have and, you multiply. So the probability of having a 1st class product is the product of all of these:

$$P_A= 0.6 \times 0.5 \times 0.9=0.27=27 \%$$

Same exact thing for B, except of course the probabilities are different (and there's another step). We want the probability of:

  • First step goes well ($P=1-0.1=0.9$) AND
  • Second step goes well ($P=1-0.2=0.8$) AND
  • Third step goes well ($P=1-0.3=0.7$) AND
  • It's the chose one ($P=0.7$)

Again, multiply them all:

$$P_B = 0.9 \times 0.8 \times 0.7 \times 0.7 =0.3528=35.28\%$$

Therefore, technology B has a higher probability of giving a 1st class product.

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  • $\begingroup$ WOW, thanks a lot, you saved me, i would give you 50 thumbs up if i could. This is so simple, but I'm too tired, just too tired... Sorry... $\endgroup$ – Guest551469 Apr 21 '14 at 21:09
  • $\begingroup$ @Guest551469 No problem, good luck~ $\endgroup$ – Shahar Apr 21 '14 at 21:22

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