# What is Probability of Compound Events?

Feb 2, 2015

It matters whether the events are independent or not.

Example
If you roll a die twice, the second roll is independent of the first, because a die doesn't have memory.

Probabilty of rolling two sixes
First: $P \left(1 s t = 6\right) = 1 / 6$
Second: $P \left(2 n d = 6\right) = 1 / 6$

Probability of two sixes means:
First is 6 AND second is 6:
$P \left(1 s t = 6\right) \cdot P \left(2 n d = 6\right) = 1 / 6 \cdot 1 / 6 = 1 / 36$

Probability of rolling more than 4 (i.e. 5 OR 6) in one try:
$P \left(5\right) = 1 / 6$
$P \left(6\right) = 1 / 6$
$P \left(> 4\right) = P \left(5\right) + P \left(6\right) = 1 / 6 + 1 / 6 = 1 / 3$

General rule
If it's an AND situation multiply the chances
If it's an EITHER.. OR situation, you add