The formula for combinations is $$ \frac{n!}{r!(n-r)!} $$ where n is the total number of items and r is the number of items being chosen.
So, in this case, the expression evaluates to $$ \frac{5!}{3!(5-3)!} = \frac{5*4*3*2*1}{3*2*1*2*1} = \frac{120}{6} = 20 $$
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