You can put this solution on YOUR website! choose 7 people from 20 --> simple combinatorial problem:
(20 c 7) - read "twenty choose 7"
(20 c 7) = 20*19*18*17*16*15*14/(7*6*5*4*3*2*1)
= 77,520
If you need more help go to http://www.mathsisfun.com/combinatorics/combinations-permutations-calculator.html