GRE数学考点：AT least One

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　　AT least One

　　If probability of event A happening is p and event B happening is q, then
the probability that at least one of them happens is (p+q).

　　Example:

　　There are 20 quarters, 33 nickels, and 47 pennies in a jar. The probability
that a randomly selected coin is a quarter= 20/(20+33+47)=20/100=0.2 and the
coin is nickel = 33/100 =0.33.The probability it is either a quarter or a nickel
= 0.2+0.33=0.53.

　　Remember:

　　The events needs to be mutually exclusive, that is if one happens the other
cannot happen: a coin is either a quarter or nickel but not both.