A bag contains seven balls of the same size, including three red balls, two black balls and two white balls, Take 2 balls continuously from them (do not put them back after taking them) (1) The probability of getting the red ball for the second time

A bag contains seven balls of the same size, including three red balls, two black balls and two white balls, Take 2 balls continuously from them (do not put them back after taking them) (1) The probability of getting the red ball for the second time

Method 1
This situation is equivalent to the probability of seven people touching a ball and the second person touching a red ball. In fact, the probability of each person touching a red ball here is equal. Because we don't know whether the first ball is a red ball, we need to treat it as equivalent, that is, the probability of touching a red ball for the second time is 3 / 7
Method 2
If you get a red ball for the first time, the probability of getting a red ball for the first time is 3 / 7. If you want to get a red ball for the second time, the probability is 2 / 6. Therefore, the probability of this case is 3 / 7 × 2 / 6 = 1 / 7;
If you don't get the red ball for the first time, the probability of not getting the red ball for the first time is 4 / 7, and the probability of getting the red ball for the second time is 3 / 6, so the probability of this case is 4 / 7 × 3 / 6 = 2 / 7;
The above two cases are the probability of getting the red ball for the second time, so the total probability = 1 / 7 + 2 / 7 = 3 / 7

If 18 = 228 = 238 = 248 = 258 = 368 = 288 = 498 =?
Ask for detailed explanation

Haha, this is simple. The "8" of 18, 28, 38, 48 and 58 all have two circles ("0"), which are equal to 2. There are four circles in "88", which are equal to 4. There is another circle in "6" of "68" and "9" of "98", so 98 = 3

How many gold medals can China win? 38, 48, 58, 68, 78, 88

68

Given the square of (n + 68) = 654421, then (n + 58) (n + 78)

(N+58)(N+78)
=(N+68-10)(N+68+10)
=(N+68)^2-10^2
=654421-100
=654321