Question 548939
The 1st choice is any one of 20
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There are 19 left, but one of them is 
the twin of the 1st choice, so there
are 18 valid 2nd choices
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There are 17 3rd choices, but one of these is
the twin of the 2nd choice, so there are
16 valid 3rd choices
{{{ 20*18*16 = 5760 }}}