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Suppose that a set S has n elements. How many ordered pairs(A,B) are there such that A and B are subsets of S with A$\subseteq$ B?
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Each element can either be

(1) in neither A nor B,

(2) in only B,

(3) in both A and B,

there are total n element and each element have 3 choice 

so total order pair 3n

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