High school math probability questions! Come in!!! You can add points for a good answer!

(1) from 20 hundred dollar bills in any 2, one of which is a fake bill *** there are 5 * 15 (a fake a real) + 10 (two fake) = 85 basic events, two are fake contains 10 basic events, so in one of them on the premise of the bill detector test found to be fake, the probability of two are fake is 10/85 = 2/17.

(2) from 20 hundred dollar bills in any 2, in the first for fake, the probability of two are fake is 10/85 = 2/17.

(2) from 20 hundred dollar bills in any 2, in the first for fake, the probability of two are fake is 10/85 = 2/17. p>(2) from the 20 hundred dollar bills in any two, in the first for the premise of fake bills, the second for the probability of fake bills must be 4/19 (because at this time ***19 bills, there are 4 fake, draw one of the probability of a fake is naturally 4/19)

Summary: according to the textbook, this problem is a conditional probability problem. But we can see this problem only as a probability problem in line with the classical generalization, in the conditions known (1) from 20 hundred dollar bills in any two, one of which is a fake bill (2) from 20 hundred dollar bills in any two, in the first for the premise of fake bills under the premise of the Calculate the total number of basic events and the number of basic events contained in the requested event, divide can be!