# The box contains 80 parts, 70 of which are standard. At random, two parts are extracted in turn. Find the probability

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 Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.