Conditional probability refers to the case where we have some data that could modify the probability that a certain event has occurred.

**Example from everyday life:**

If we are asked what is the probability that the first girl to leave the classroom has brown hair.

We won't know how to say.

But, if we are provided with information that in the classroom there are only $2$ blonde girls among $30$ girls in total, the probability we can present will be different.