On the principle of no return in random sampling

Of course, both can guarantee equal opportunities, but we should consider the reality! For example, five people draw lots to decide the order of speaking, and they must not be signed back, otherwise there may be two people drawing lots. Another example is 1 building. The lottery in the mall must be put back, otherwise it will be difficult to meet the needs of a large number of customers.

The answer on the second floor is wrong. If you don't put it back, the probability of drawing the No.2 ball is

The first time is not the second ball * the second time is the second ball.

That is 9/10 *1/9 =110.

The probability is the same.