Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

1-The probability of passing test A is a, The probability of [#permalink]
07 Dec 2003, 21:39

00:00

A

B

C

D

E

Difficulty:

5% (low)

Question Stats:

0% (00:00) correct
0% (00:00) wrong based on 0 sessions

1-The probability of passing test A is a, The probability of
passing test B is b, if a student participates in both tests what is
the probability of passing either A or B but not both ?

I didn't know the formula, but I plugged a few numbers and got #2.

I'd like to see someone provide a little theoretical backing. I worry that my "pick some numbers and plug" strategy will backfire in a high stress situation.

Support Racer, P(a or b )=Pa+Pb-P(a and b), Example Pa=50%, Pb=70% P(A or B)=70%+50%-35%=85%, if it was A+B-2AB then it is 120%-70%=50% which makes no sense since the prob of passing either should be higher than the probability of passing a single exam...IMO

Draw a tree diagram , the problem would be easier to see.
Prob of Passing only A = Prob of success in A * Prob of failure in B
= a * (1-b) = a - ab
Similarly, Prob of Passing only B = b * (1-a) = b - ba
The probability of either A or B = a - ab + b - ba = a +b -2ab
its choice (2).

Say there's a 100% chance of passing test a, and a 100% chance of passing test b.

1+1-(1*1) =1

The question asks for "the probability of passing either A or B but not both?". The answer to this question is zero, if the chance of passing each test is 1.