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)?

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.

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

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