Khan/khan-exercises — pull request #126053 by vilhelmo (2013-11-25 06:07:48+00:00)
Title: Created exercise for Addition rule for probability

Create a few basic exercises for the addition rule for probability (https://www.khanacademy.org/math/trigonometry/prob_comb/addition_rule_prob_precalc/v/addition-rule-for-probability)