Find the probability and talk about it in detail.

Take the exam four times, in order, so the questions should be calculated in order.

It can also be understood as taking four times in turn, which is equivalent to taking four balls randomly from 10, and then all the arrangements of these four balls,

Therefore, the total number of schemes: A4/ 10, (4/ 10 respectively stands for the subscripts in the arrangement formula,)

If you want to get two red ones and the other two are white, the number of schemes for randomly selecting two from eight white balls is C2/8.

And the number of these two red and two white arrangements is A4/4.

So I got two red schemes: C2/8*A4/4.

So we get the probability of two reds: the number of schemes with two reds/the total number of schemes = (C2/8 * A4/4)/(A4/10) = 2/15.

Your supplementary question and the original question are two different questions, right?

This situation is more complicated and can be divided into six situations:

(1) gets the red ball for the second time at 1, and the red probability of 1 is 2/ 10, and the red probability of the second time is110.

p 1 = 2/ 10 * 1/ 10 = 200/ 10000

(2) 1, the red ball is obtained for the third time, the red probability is 2/1 0 for the first time, the white probability is 9/ 10 for the second time and the red probability is110 for the third time.

P2 = 2/ 10 * 9/ 10 * 1/ 10 = 180/ 10000

(3) 1, get the red ball for four times, 1 get the red probability 2/ 10, get the white probability 9/ 10 for the second time, get the white probability 9/ 10 for the third time, and get the red probability/kloc-0 for the fourth time.

P3 = 2/ 10 * 9/ 10 * 9/ 10 * 1/ 10 = 162/ 10000

(4) Get the red ball for 2 or 3 times, 8/1 0 for the first time, 2/ 10 for the second time, and110 for the third time.

P4 = 8/ 10 * 2/ 10 * 1/ 10 = 160/ 10000

(5) Get the red ball for 2 or 4 times, 8/1 0 for the first time, 2/ 10 for the second time, 9/ 10 for the third time and10 for the fourth time.

P5 = 8/ 10 * 2/ 10 * 9/ 10 * 1/ 10 = 144/ 10000

(6) 3,4 red, 1 white 8/ 10, second white 8/ 10, third red 2/ 10, fourth red110.

P6 = 8/ 10 * 8/ 10 * 2/ 10 * 1/ 10 = 128/ 10000

p = p 1+p 1+P3+P4+P5+P6 = 974/ 10000 = 487/5000

Promise me on my honor that this is the standard answer! ! ! !