Binomial Prob passing, question, although, guess

optiix

I was thinking about this had a question

If you were to guess on every exam question (and assume that there are 4 answer choice and not 3 although we could solve it either way).

The probability of guessing 70/100 questions correct would by 100C70*.25^70*.75^(100-70).

Lets say 70% is a pass. Is the probability of passing then 100C70*.25^70*.75^(100-70) + 100C71*.25^71*.75^(100-71)+.......100C100*.25^100*.75^(100-100)?

kmf229

no......the binomial we learned was to find the exact probability of choosing x out of n. I was trying to find the probability if choosing at least x (or more) out of n, which is different and hence the question.

bapswarrior

use minitab CDF function (if u r familiar with program) or u will have to count probability of exact result for each one and then sum them up - hassle

disiz64

Your formula is correct (this is easy to verify e.g. in Excel). The answer is 0.43*10^(-20).



Edited 1 time(s). Last edit at Wednesday, December 8, 2010 at 01:38PM by shootingstar.

lc26mizzou

yeah i tried finding probability of passing cfa level 1 assuming 65% passing rate by guessing entire exam - it was 0

soddy1979

I think the formula is the sum of the probabilities of getting 70/100 + 71/100 + 72/100...100/100, where the (first) formula is .25^70*.75^30 and on and on.

kd26gioi

There are 240 questions, not 100. The odds are zero.