概率论的题目题意不清（英语）Therearethreetypesofanimalsinalaboratory:15typeI,13typeII,and12typeIII.AnimalsoftypeIreacttoaparticularstimulusin5seconds,animalsoftypesIIandIIIreacttothesamestimulusin

Question

概率论的题目 题意不清（英语）
There are three types of animals in a laboratory:15 type I ,13 type II ,and 12 type III.Animals of type I react to a particular stimulus in 5 seconds,animals of types II and III react to the same stimulus in 4.5 and 6.2 seconds ,respectively.A psychologist selects 10 of these animals at random and finds that exactly four of them react to this stimulus in 6.2 seconds.What is the probability that at least two of them react to the same stimulus in 4.5 seconds?
A psychologist selects 10 of these animals at random and finds that exactly four of them react to this stimulus in 6.2 seconds.这句话的意义何在 不是抽样的每一只animal都会在6.2s内反应吗.这道题书上没有给答案.只求思路）

Answer 1

不是,是抽取10只,当且仅当4只的反应时间恰好是6.2s,（也就是说第三类恰好4只）这道题目是让你求条件概率.
题目大意：三类动物各15,13,12只,从中随机抽取10只,第三类动物恰好有4只,问在这种情况下第二类动物至少两只的概率.