Complementary counting
Revision as of 20:53, 28 February 2017 by Expilncalc (talk | contribs) (Added another problem which contains this technique)
Complementary counting is counting the complement of the set we want to count, and subtracting that from the total number of possibilities, or the universal set for that particular problem. A big hint that complementary counting may lead to a quick solution is the phrase "at least".
