math
posted by susan
A truefalse test consists of 15 items.
(b) What is the probability that Chris gets 80% or more for the test?
(c) If it is a 25 item truefalse test, would you think it is easier or more difficulty for Chris to get 80% or more? (Hint: please calculate the corresponding probability that Chris gets 80% or more for the test, and compare your results with the one obtained in (b) to answer this question.)
Answer.
b) he must get 13 right or 14 right or 15 right
= C(15,13)(1/2)^13 (1/2)^2 + C(15,14)(1/2)^14(1/2) + C(15,15) (1/2)^15
= 0.01922%
c) he must get 23 right or 24 right or 25 right
= C(25,23)(1/2)^23 (1/2)^2 + C(25,24)(1/2)^24(1/2) + C(25,25) (1/2)^25
= 5.36441803e0.5

Reiny
The assumption would be that he would be randomly guessing at the answer.
Since 12/15 = .8, why did you not include the probability of getting 12 correct out of the 15 ?
Try adding that to your other cases.
For b) wouldn't getting 20 out of 25 correct yield 80% ?
Same for 21 and 22
So you have a few more cases to include 
lol
maybe speak to your tutor if you're having issues. very easy to get done for plagiarism for this when it shows up as the first result on google. QUT is coming for you
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