n>_4, altfel combinarile nu au sens, ''>_''=mai mare sau egal
(n!)/[4!x(n-4)! =5n(n-3)/6
n(n-1)(n-2)(n-3)/24=5n(n-3)/6
n(n-1)(n-2)(n-3)/4=5n(n-3)
n(n-1)(n-2)(n-3)=20n(n-3), impartim prin: n(n-3)
(n-1)(n-2)=20
n^2 -3n +2-20=0
n^2 -3n -18=0
(n+3)(n-6)=0
n-6=0
n=6
