The box contains 80 parts, 70 of which are standard. At random, two parts are extracted in turn. Find the probability that both extracted parts are standard if:
1) the first part is returned back to the box before removing the second;
2) the first part is not returned to the box.
Consider the events:
A – the appearance of a standard part in the first test;
B – appearance of a standard part in the second test.
Then AB – both parts are standard.
1) In the first case, events A and B are independent and P = 70/80 * 70/80 = 0.77
2) In the second case, events A and B are dependent. If A happened, then 79 parts will remain in the box, and there will be 69 standard parts and then P = 69/79 = 0.87
Then 70/80 * 69/79 = 0.76
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