Probability Riddles

If you put a single precious black stone in one bag and all the others in the other bag, it will give you almost three/fourth of the probability of picking up a precious black stone.

The probability will be 1/36.

For the first dice, there can be six possibilities. Similarly, for the second dice as well, there can be six possibilities.
Thus the total possibilities is 6 * 6 = 36.
The outcome we need is that the first comes up with a 2 and the second comes up with a 5. That is possible only in one possibility.
Therefore the required probability is 1/36.

Let us take this one step at a time.

Let us calculate the probability that Anna and Brian have the same number in their mind.
101/125

Now, there's a 1/5 chance that the numbers will be same and 4/5 chance that the numbers are different.

Let us now include Christy in this data. There can be two cases.
1) Anna and Brian have the same number. In that case, Christy will have only one number to compare.
2) Anna and Brian did not have the same number. In that case, Christy will have two numbers to compare to.

For the first case, the probability will be 5/25. This is if Anna and Brian did have the same numbers.

But if Anna and Brian did not have the same numbers, there is a 2/5 probability that Christy is having the same number (this is because Christy gets to match her number with both Anna and Brian). In that case, we can simply multiply the probabilities.
4/5 * 2/5 = 8/25

Otherwise, if Christy is not having the same number, the probability is 3/5. Now multiplying with the previous chain:
4/5 * 3/5 = 12/25

Now, we can include Drake in our calculations. If we follow the path where Drake's number matches with Anna and Brian, the probability will be 25/125.

Now let us join that with Christy's probability. If Christy's number matches with Anna and Brian and Drakes' number also matches, then the probability will be:
4/5 * 2/5 = 40/125

If Christy's number does not match with Anna and Brian but Drake's number matches with Christy's, the probability will be:
4/5 * 3/5 * 3/5 = 36/125

But if Christy's number does not matches with Anna and Brian and even Drake's does not matches with Christy, then the probability will be:
4/5 * 3/5 * 2/5 = 24/125

Now, we have to tell the probability when all the four friends have same numbers, so we will just add up the probability where all the numbers matches:

25/125 + 40/125 + 36/125 = 101/125.

50%

The next toss of the coin is not dependent upon the previous ten tosses. Thus, the probability is 50 percent only.

This question can be a tricky one. The probability that is asked will be zero.

If the probability off taking out a pair of white socks is 1/2, it means that there is no black pair. This is because if there had been, there would have been three cases – white pair, black pair and mixed pair. But since in the question it is clear that there were just two chances (1/2), we can fairly deduce that there were only one black sock and the rest were white.

1) If bullets are placed in the adjacent chambers:
Let us do this step by step. The possible combinations for the bullets are:
1, 2
2, 3
3, 4
4, 5
5, 6
6, 1

Now if you ask not to spin the barrel:
The first shot went empty and thus the events (6, 1) and (1, 2) dont have bullets in them. Now only four possible shots are available.
Thus, P (death) = 1/4 = 0.25
P (Survival) = 1- 0.25 = 0.75

If you ask to spin the barrel:
Since you were not hit in the first shot, therefore
P (death) in the second shot = 2/6 = 0.33
P (Survival) in the second shot = 1 - 0.33 = 0.77

Clearly, you will have better chances of survival if you ask him not to spin the barrel.


2) If bullets are not placed in the adjacent slots:

If you dont spin it:
You were not hit in the first shot. Thus,
P (Death) in second shot = 2/5 = 0.4
P (Survival) in second shot = 1 - 0.4= 0.6

If you spin the barrel
P (death) in the second shot = 2/6 = 1/3 = 0.33
P (Survival) in second shot = 1 - 0.33 = 0.77

Here, you have better chances of survival if you chose to spin the barrel.

1/2

In the house on the left, there are three possibilities:
Younger Older
Girl Boy
Boy Girl
Boy Boy

We cant have a girl-girl option because it has been mentioned that there is at least one boy in that house. Now all these outcomes are equally likely, and we have two events with girls, the chances of having a girl in the left house is 2/3.

In the house on the right, there are two possibilities because we already know that the older child is a boy:
Younger Older
Girl Boy
Boy Boy
Here the chances are 1/2.

Thus you must choose the house on the left for better chances at winning.

For best probability of living, you must place one white pearl in a bowl and all the rest into another bowl.

This gives you a 1/2 probability of choosing a bowl which has just a white pearl. If you choose that, you will live.
If you choose another, you will still have an almost 1/2 probability of picking up a white pearl. (49 white pearls and 50 black pearls).

2/3. How? Well, since he has 3 arrows, each shot has 1/3rd chance of being the best shot. So the chance that the third shot is not the best shot is 1 minus 1/3rd, thus 2/3rd.

the person C will have the highest chances of survival.
Person A hits = 1
Person A misses = 0
Person B hits = 4/5
Person B misses = 1/5
Person C hits = 1/2
Person C misses = 1/2