There are 10 new balls and 2 old balls in the bag. If you take one at a time and don't put it back, what is the probability of taking out the old ball for the second time? My probability is very poor ~
Principle of drawing lots: it is equivalent to 10. Two of the lottery tickets are winners, and each person draws one at a time. No matter whether you draw first or later, everyone's lottery winning probability is equal to 2/12 =1/6;
A full row is a full row, that is, twelve balls are arranged in a row, that is, A( 12, 12), and the old ball is in the second position. There are two ways to place the remaining eleven balls randomly, so the probability is 2 * A (1 1, 65438+).
I don't know how to explain this. You got it?