Whenever the questions asks for the probability and has "or" you have to add the two probabilities. In the same way, "and" implies multiplication. Therefore, the probability that someone swims is:
Probability that someone runs:
Thus, if you add them:
So, the probability that someone swims <em>or</em> runs is 1/2 or 50%.
Probability that someone runs:
Thus, if you add them:
So, the probability that someone swims <em>or</em> runs is 1/2 or 50%.