A sample space has four outcomes with probabilities 0.1, 0.3, $p$ and 0.2. What is $p$?

Explanation

1. 01

   The probabilities of all the outcomes of a sample space add up to 1.

2. 02

   $0.1 + 0.3 + p + 0.2 = 1$, so $p = 0.4$.