Use the standard formula for the number of combinations

     =  = .


Reduce common factors 16! in the numerator and the denominator.  The formula takes the form

     =  = use your calculator = 4845.      ANSWER

Start with 20, go down 1 at a time multiplying until you have 4 factors
on top.

Then there are also 4 factors on the bottom, starting with 4, going down 1
at a time until you have 4 factors on the bottom. 


like this:




Then figure that out.

Edwin