http://www.ck12.org Chapter 2. Conditional Probability
These are independent events, since the outcome of tossing the first coin in no way affects the outcome of tossing
the second coin. Therefore, we can calculate the probability as follows:
P(AandB) =1
2
×
1
2
P(AandB) =1
4
Therefore, we can conclude that the probability of getting 2 heads when tossing a coin twice is^14 , or 25%. Let’s try
an example that is a little more challenging.
Example B
Irvin opens up his sock drawer to get a pair socks to wear to school. He looks in the sock drawer and sees 4 red
socks, 6 white socks, and 8 brown socks. Irvin reaches in the drawer and pulls out a red sock. He is wearing blue
shorts, so he replaces it. He then draws out a white sock. What is the probability that Irvin pulls out a red sock,
replaces it, and then pulls out a white sock?
First let’s draw a tree diagram.
There are 18 socks in Irvin’s sock drawer. The probability of getting a red sock when he pulls out the first sock is:
P(red) =4
18
P(red) =