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10 different names are put into the computer and one of the name is Colin.

a) On Saturday, the computer chooses 2 names at random. The computer is set so the same name *can* be chosen twice. What is the probability that Colin is chosen at least once?

b) On Sunday, the computer is set so that the same name *cannot* be chosen twice. Work out the probability that Colin is chosen.

 Mar 19, 2020
 #1
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a)  Colin's name can be chosen on the first try but not on the second:  (1/10) x (9/10)  =  9/100

     or his name can be chosen on the second try but not the first:         (9/10) x (1/10)  =  9/100

     or his name can be chosen on both tries:                                         (1/10) x (1/10)  =  1/100

     The answer is the result of adding these three values.

 

b)  Colin's name can be chosen on the first try but not on the second:  (1/10) x (9/9)  =  1/10

     or his name can be chosen on the second try but not the first:         (9/10) x (1/9)  =  1/10

     Again, the answer is the result of adding these two valus.

 Mar 19, 2020

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