由题可知 C n 3=C n 7所以根据组合数规律知n=3+7=10
nC3 =nC7n(n-1)(n-2)/6 = n(n-1)(n-2)(n-3)(n-4)(n-5)(n-6)/5040(n-3)(n-4)(n-5)(n-6)=840n=10
