If I have A2 numbers in a pot and I select B2 of them, then draw out C2, how
do I write a formula that will calculate the chance of the number of numbers
drawn in C2 matching (some or all, depending on whether B2 = C2, but B2 is
always = C2) the number of numbers in B2?

Example 100 balls, 100 to match, 100 drawn - chances are 1:1 that there will
be a match - or just displayed in an cell 1.

I know how do do the chance that A2 ball in B2 will match by using
=COMBIN(B2,A2) but thats different to what I'm asking.