interpunct?? what is this?

The exclamation mark is the factorial sign.  So 21!, spoken as twenty-one factorial, means 21*20*19*...*3*2*1,

$$\frac{21!}{5!6!10!}=\frac{21*20*19*...*11*10*9*...*2*1}{5!*6!*10*9*...*2*1}=\frac{21*20*...*11}{5*4*...*2*1*6*5*...*2*1}\\\\=\frac{14079294028800}{120*720}=\frac{14079294028800}{86400}=162954792$$

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