请教两个JJ题目

  4月JJ438题：438 ps. 1/6 the student of the school at least know how to play one instrument,86 student of the school know how to play two or three instruments, there are 800 at school , if you choose 3 students out , what is the possibility that the 3 students exactly play one instrument? (those statistis are not exactly the same as the original question on the screen)

   我的计算是：800*1/6-86=A个

   那么概率是C（3，A）/ C（3，800），和HORSEFISH的答案不一致，horsefish评论: 1/236
思路: 3/(800*1/6-86)     请NN指点一下

  另310题：310 ps。10个人进一饭店，但店里只剩四把椅子，问怎么做法？(本月出现两次)

   我觉得就是从10个人中选4个人，C（4，10），有人做P（4，10），不是很理解，请教

  4月JJ438题：438 ps. 1/6 the student of the school at least know how to play one instrument,86 student of the school know how to play two or three instruments, there are 800 at school , if you choose 3 students out , what is the possibility that the 3 students exactly play one instrument? (those statistis are not exactly the same as the original question on the screen)

   我的计算是：800*1/6-86=A个

   那么概率是C（3，A）/ C（3，800），和HORSEFISH的答案不一致，horsefish评论: 1/236
思路: 3/(800*1/6-86)     请NN指点一下