Question

Suppose you have 6 pairs of socks in your sock drawer. It is still dark out in the morning when you get dressed, so you just pull socks out of the drawer at random, one at a time, until you have removed two matching socks. What is the probability that you pull out exactly 5 socks from your sock drawer in the morning?

Answer 1

Answer:

8/33 = 0.242

Step-by-step explanation:

There are 12 socks to begin with and one is taken;

The second is picked from the remaining 11 and will either match the first or not;

The probability of matching is 1/11 and 10/11 for not matching;

If the sock matches, no further sock is taken;

If the socks don't match, a 3rd sock is taken;

The 3rd sock is taken from the remaining 10, 1 of which will match with the first sock and another which will match the second;

Therefore, the probability of taking a matching sock on the third pick will by 2/10 or 1/5 and the probability it will not match either of the first two is 8/10 or 4/5;

Once again, if the sock matches either of those previously taken, no further sock is taken;

If the socks don't match, a 4th sock is taken;

Now, there are 9 socks remaining in the drawer;

Picking is repeated;

The process is logical;

The attached picture shows a tree diagram representing this information in an easily digestible way and makes it easy to work out the desired probability;

With tree, diagrams, you just multiply across until you get to your desired outcome;

In this case, we are finding the probability of picking 5 socks exactly;

Each process splitting represents the picking of a sock;

The probability will be:

10/11 × 8/10 × 6/9 × 4/8 = 8/33 = 0.242...