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.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

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:

I take advantage of this first post to say hi to the GMAT Club which I find very interesting and useful.

I experience some difficulties with a simple probability problem found in the book "Conquering GMAT Math and Integrated Reasoning":

"What is the probability that a card selected at random from a standard deck of playing cards will be a 7 or a club ?"

It's a example to illustrate some theory so there is no multiple choice but the answer is: "There are 52 cards in a deck. There are 13 each in hearts, clubs, spades, and diamonds. There are four 7s, one each in hearths, clubs, spades, and diamonds. Thus the 7 of clubs is counted twice , once as a club and once as a 7 There are 13 clubs plus tree 7s that are not clubs. P(7 or Club) = 16/52 = 4/13

My problem with this answer is actually the fact that this 7 of clubs isn't counted twice as announced : why doesn't this card count as a double chance of getting an expected card (7 or club) ? My natural answer would have been P= (13+4)/52 = 17/52

I honestly don't know a great explanation for you other than use logic. You should not and cannot double something in a probability...at least in this setup. Just because a card meets multiple criteria it does not mean it needs to be counted multiple times.

The trick that they want you to catch on to is the fact that you CANNOT count the 7 of clubs twice.

It is by definition that Probability of an event = (number of favorable cases)/(exhaustive number of cases)

The number of favorable cases would be 16, 'cause even if 7 of club is picked - although it fulfills the criteria of being a club or being 7, it will be counted as 1 card only.

And therefore the probability is 16/52.

So, whenever dealing with probability questions, try to visualize a real life example...and it will make the problem easy.

Regards,

gmatclubot

Re: Another Cards Problem
[#permalink]
27 May 2012, 06:06

Hey, guys, So, I’ve decided to run a contest in hopes of getting the word about the site out to as many applicants as possible this application season...

Whether you’re an entrepreneur, aspiring business leader, or you just think that you may want to learn more about business, the thought of getting your Masters in Business Administration...

Whether you’re an entrepreneur, aspiring business leader, or you just think that you may want to learn more about business, the thought of getting your Masters in Business Administration...