A set of n elements has 2n subsets. Our set has 4 subsets. Thus, we have the equation
2n =

That should be 2^n and not just 2n

If we have 4 subsets, then 2^n = 4 leads to n = 2 since 2^2 = 4.

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Here's an example.

A = {p,q}

The four subsets of set A = {p,q} are

{p,q} ... any set is a subset of itself{p} .... a set with one element inside it is called a singleton set{q} ... also a singleton set{ } ... the empty set, you can use the special symbol to denote the empty set

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Here's an example with n = 3 elements. We'll have 2^n = 2^3 = 8 subsets now. Let's say we had B = {r,s,t}

Those 8 subsets of set B are...

{r,s,t}{r,s}{r,t}{s,t}{r}{s}{t}{ } or

Note that order does not matter within any given set. Something like {r,s} is the same as {s,r}.

Subtracting is the opposite of adding

The first one is 2/3the second is 1/160the third is 4/125