If we choose 4 people randomly, find the probability (in %)

Question

If we choose 4 people randomly, find the probability (in %) that none of their birthdays fall on the same day of the week

(a) 23 
(b) 35 
(c) 38 
(d) 27 
(e) None of the foregoing

sudhir18n · Answer

7P4/7^4
7*6*5*4/7*7*7*7
120/343 = 
hence B

hussi9 · Answer

Solution

We can choose 4 different days out of 7 days in 7C4 ways. Now these 4 days can be given to 4 people in 4! ways.
So possible ways are = 4! * 7C4
Total no of ways is 7*7*7*7(all 4 can have any day as B'day) = 7^4
Probability Percentage is  *100 = 35
Hence, choice (b) is the right option

hussi9 · Answer

7P4/7^4
7*6*5*4/7*7*7*7
120/343 = 
hence B[/quote

Your solutions seems simpler

sudhir18n · Answer

Whats the source of your questions?

hussi9 · Answer

I have some question bank for IIM CAT , Indian MBA Exam.

sudhir18n · Answer

IIM CAT is supposedly notches ahead of GMAT Quants. If I were you I would avoid doing LOD 2/3 questions of CAT.
Its good to know the concept and use it but what I know and hear is that probability questions are very very rare on GMAT.

All the best

hussi9 · Answer

I am just going though Question to get a taste of different types of questions... So that there are on surprises on GMAT.
Also it feels good...

But I get your point ... I will put less of probability questions...

gurpreetsingh · Answer

I think this question was not that tough for Gmat. But be vary of IIM questions. Instead of wasting your time to improve q50 to q51 , better improve verbal.

Increment in verbal score improve the overall score more than the same in quant score.

For quant, DS is the tricky part. So make sure you do spend considerable time on it. PS won't give you much surprises.

hussi9 · Answer

I think thats great advice ... Will keep that in Mind....

jamifahad · Answer

Ya good advice gurpreetsingh. 
I was wondering about how much effort should i give to probability? I am not all that comfortable with probability..often mix up few conceptual stuff. Should i master probability or just be able to solve elementary level questions on probability?

gurpreetsingh · Answer

you should definitely study probability and combinations, but do not spend too much time on them. I think 1-2 days should be sufficient.

KarishmaB · Answer

People see 1-2 Probability and Permutation Combination questions on GMAT. If you are doing exceptionally well, you might see 2-3. This is so because these questions can be a little tricky, in the sense that you may not even doubt your answer but it could still be wrong. Possibly, one of the questions will be simple, one will be trickier. 
As gurpreetsingh said above, if you have limited time, focus more on Verbal. If you do have lots of time, go ahead and freak out on Probability. At least the questions are brain teasers!

hussi9 · Answer

Using my critical reasoning skills I infer that it makes sense to ponder on P&C question only if you consistently cross 700+ mark

Am I even close to what you trying to say ?

KarishmaB · Answer

Well, yes it depends on where you are in the rest of the topics since they certainly have more weightage. If you are good with the others, then it makes sense to worry about tough P&C questions. It also depends on how much time you have. If you have plenty of time, once you are done with Arithmetic, Algebra and Geometry, you can spend more time on P&C.

That said, everyone should be ok with the basic P&C so that you can attempt any easy P&C question you might get.

hussi9 · Answer

@Karishma
Well then I surely have saved myself some time by posting this question.

Thank you so much. And by the way I like you blogs. Not gone though all of them but the topics are interesting

IanStewart · Answer

Probability and Counting is just like any other subject in GMAT math: it can be easy and it can be hard. There's a myth I often see repeated in forums that you're more likely to see Probability/Counting questions at the high level of the test, and that just isn't true. If I'm remembering correctly, I had one question in the area on my last test, for example. Sometimes people see a few questions on the topic, but it's rare that it's heavily tested.

You certainly want to know the basics in the subject (which can be learned quite quickly), because you don't ever want to run the risk of answering low-level questions incorrectly on the GMAT. But using IIM questions for practice is utter overkill; it would be like using an advanced calculus book to study for GMAT algebra. Interesting as those IIM problems may be, very little of what you will learn from them will ever be applicable on the GMAT, and it's vastly more efficient just to study from dedicated GMAT materials.

jamifahad · Answer

Thanks everybody for sharing wonderful insights.

If i may, i want to know how can one increase speed? I can pretty much solve all the problems but i am plagued by timing. if i hurry up i always goof up with calculations. 
Sorry for asking this at wrong place. Any pointers will be appreciated.

amit2k9 · Answer

birthdays on different days = 7*6*5*4
total possibilities = 7*7*7*7

hence 7*6*5*4/ 7*7*7*7 = B

seaplane · Answer

7/7*6/7*5/7*4/7 = 120/343

If we choose 4 people randomly, find the probability (in %)

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If we choose 4 people randomly, find the probability (in %)

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