Question 171110
2nd) P(8 correct) is  (15(nCr)8)(5(nCr)2)  / 20(nCr)10
3rd) P(at least 9)=P(9 correct)+P(10 correct)
=[(15(nCr)9)(5(nCr)1)+ (15(nCr)10)] / 20(nCr)10