要选多少个?
C(44,7)/C(49,7)=44*43*42*..*38/(49*48*...*43)=42*41*40*39*38/(49*48*47*46*45)=41*13*19/(7*47*23*3)=10127/22701≈0.44610369587242852737764856173737≈44.61%