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There are three types of animals in a laboratory: 15 type
Chapter , Problem 21(choose chapter or problem)
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?
Questions & Answers
QUESTION:
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?
ANSWER:Step 1 of 2
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.
The remaining six animals don’t react to this stimulus in 6.2 seconds. Now, in the remaining six animals, at least two of them react to the same stimulus in 4.5 seconds.
Therefore, it is enough to find out the probability that at least two animals are of type II.
The remaining six animals are either type I or type II. There are a total 28 animals of type I and type II. Six animals can be chosen is 
Therefore, the number of points in the sample space is