Question 262664
Hint: Use the combination formula (since order doesn't matter) *[Tex \LARGE n^{C}r = \frac{n!}{r!(n-r)!}]



In this case, {{{n=20}}} (he has 20 to choose from) and {{{r=4}}} (but in the end he can only pick 4) which means that *[Tex \LARGE 20^{C}4 = \frac{20!}{4!(20-4)!}]



Note: computing 20! isn't practical. Try to see if you can think of a way to simplify it first before plugging it into your calculator.