Question

In a sample of 10 telephones, 4 are
defective. If 3 are selected at random
and tested, what is the probability that
all will be non defective?​

Answer 1

Answer:

Imagine picking the phones one at a time. As you mentioned, the probability the first phone picked is OK is 6/10. Given that this happened, the probability the second phone is OK is 5/9, since there are only 5 good phones left.

Thus our probability is 6/10⋅5/9. This product can be simplified.

♥How many ways are there to choose 2 telephones to test out of the 10 telephones in the sample?

♥How many ways are there to choose 2 non-defective telephones out of the 6 non-defective telephones in the sample?

The answers to both of the above can be computed using binomial coefficients. Now, divide the answer to question 2 by the answer to question 1 to get the probability of picking 2 non-defective telephones...✅

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Answer 2

Answer:

since no. of possible outcome is 6 and 3 telephones are selected probability = 3/6=1/2