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March 2, 2015

March 2, 2015

Posted by **Anonymous** on Friday, February 28, 2014 at 7:59pm.

a. What is her probability of each of the following?

1. Getting every note right.

My answer: 0.547

I am stuck on problems 2-5 because when I set up the nCx I got 0 for all the answers which is not sensable.....

2. Making exactly one mistake

3. Making exactly two mistakes

4. Making at least two mistakes

5. Making more than two mistakes

b. What must be Clara's probability of getting any one note right if she wants to have a 95% probability of getting all 60 notes right?

Is this the correct set up ?

x^60=0.95

My answer: 0.9991454771

- Probability -
**Reiny**, Friday, February 28, 2014 at 8:39pm#1 is correct, in effect what you are doing is

C(60,60) (.99^60 (.01)^0 = .547

#2, you want to have 59 correct and one wrong

= C(60,59) (.99)^59 (.01)^1

= 60(.55268..)(.01) = .33161

#3 , same way, 58 correct, 2 wrong

prob = C(60,58) (.99)^58 (.01)^2 = .09881

#4 , at least 2 mistakes

---> making 2 mistakes + making 3 mistakes + .. + making 60 mistakes

or 1 - making none - making 1

= 1 - .547-.33161 = .12139

#5 , making more than 2 mistakes

---> making 3 + making 4 + ... + making 60

how does that differ from #4 ?

b) to get a 95% we must have

x^60 = .95

take 60th root

x = .95^(1/60) = .999145

I think your problem is in evaluating something like

C(60,58)

by definition this is 60!/(58!2!)

perhaps your calculator cannot carry those large numbers and overloads

- most calculators have a function nCr

on mine it is found along the "5" key and my keystrokes are:

60

2ndF

5

58

= to get 1770

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