I don't understand what I'm doing wrong here? (Binomial Distribution)
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I don't understand what I'm doing wrong here? (Binomial Distribution)

[From: ] [author: ] [Date: 11-05-11] [Hit: ]
25^10)(0.The 10 C 9 thing is 10 combination 9, btw.I keep getting (what I think is) some outrageous number.2.9563.......
Here's the problem:

On a quiz of 10 questions, a student guesses on all of them. Each question has 4 possible answers, with only 1 being correct, and each is independent of every other question.

What is the probability of guessing more than 8 correctly? P(x > 8)


======================================…

This is what I did...
number of trials (n) = 10
probability of success (p) = 0.25
number of successes (x) = 9 and 10

P(9) + P(10) = (answer)

This is the equation I used

[(10 C 9) (0.25^9)(0.75^1)] + [(10 C 10)(0.25^10)(0.75^0)]

The 10 C 9 thing is 10 combination 9, btw.

I keep getting (what I think is) some outrageous number. Even when I use the calculator I get about

2.9563...which in percent form would be about 295.64%

How can the probability of something like getting 10 out of 10 questions correct on a test be 295%??

-
[(10 C 9) (0.25^9)(0.75^1)] + [(10 C 10)(0.25^10)(0.75^0)]

= 2.95639e-5
--------------------

i don't see anything wrong in the formula.
must be something to do with inputting into your calculator.
try simplified version with 10*.25^9*.75 + .25^10 = 2.95639e-5

your AD
======
2.95639e-5 means 2.95639 x 10^-5 = 0.0000295639 or 0.00295639%

-
The probability of guessing all ten correctly is 0.25^10
The probability of guessing nine of the ten correctly is 10 * 0.25^9*0.75

Hence, the probability of guessing more than 8 (i.e 9 or 10) is 0.25^10 + 10 * 0.25^9*0.75
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