actuary notes[4]

文章目录

• probability
• references

probability

1. there are ten students stand before you ,each student has unique ID in range between 1 to 10,let three students whose ID are 1,2,3 seperately Stay together and other seven students stay without order, what probablity does that arrangement happen?
   Above all, the three students can be regard as a part of those student , all students can be placed in eight position include that part and the other seven students; so the number of permutations with that arrangement is 8 ! 3 ! 8!3! 8!3! and the number of whole samples is 10 ! 10! 10!.
   according to the classical probability,the probability of that arrangement is 8 ! 3 ! 10 ! \frac {8!3!}{10!} 10!8!3!.

references

3. 《数学》