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A detailed tutorial on two-way counting. Step by step tutorial including several examples of two-way counting for reference.

Two-way counting is when any expression for a given quantity are determined using two different counting approaches. Many people believe that a quadratic equation is the perfect example of two-way counting, because you find the quantity in more than one way. However, this is incorrect. Two-way counting is actually a backwards method – you have the quantity already, you just need to figure out how you could get it. This is used often in combinations and permutations, where you often already know what quantity you need to have, you just have to figure out how to get there.