pkhats wrote:
Q. Is the probability that Patty will answer all of the question on her chemistry exam correctly greater than 50%?
1) for each question on the chem. exam, patty has a 90% chance of answering the question correctly.
2) There are fewer than 10 questions on patty's chem. exam.
Note that conceptually the question is fine for GMAT but the calculations involved make it unsuitable. You can use approximation but I don't think GMAT will give you such a question.
P(Answering all questions correctly) = P(Answering each question correctly)^n
where n is the number of questions.
Say if there are two questions, P(Answering all questions correctly) = P(Answering first question correctly) * P(Answering second question correctly)
1) for each question on the chem. exam, patty has a 90% chance of answering the question correctly.
We don't know the number of question yet. Not sufficient
2) There are fewer than 10 questions on patty's chem. exam.
We don't know the probability of answering questions correctly. We also don't know the exact number of questions. Not sufficient.
Using both together,
P(Answering all questions correctly) \(= (0.9)^n\)
We know that n is up to 9. Now check using approximation:
\((.9)^ = .81\)
\((.9)^3 = (.9)*(.8) = .72\)
\((.9)^4 = (.9) * (.9)^3 = (.9)*(.7) = .63\) (Ignore 2 of the .72 and use only .7)
\((.9)^5 = (.9)*(.6) = .54\)
\((.9)^6 = (.9)*(.5) = .45\)
Note that using approximation, \((.9)^6\) came out to be less than 50%. The powers of 9 will not deplete as fast as we have approximated since we only took the first digit after decimal. But we do get the idea that soon enough powers of .9 will go below 50%. Perhaps it will go less than 50% on the 7th power.
So depending on the number of questions, the probability of answering all questions correctly can be more than 50% or less than 50%.
Answer (E)