Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

I need a bit of help with my statistics question....because what I am thinking is correct I am thinking may not be correct...lol.

So you have 650 students.
72 of them are math majors
105 are comp Sci majors

What is the probability of a student being *both* a math and comp Sci major?

My initial thought was to add the 2 together and divide by total students.

But that just seems too simple...so I don't think it's quite right but I am not sure how to get the right answer.

Thanks friends!


Automatically Appended Next Post:
Ok..I think I figured it out...can someone double check?

Find the # of math majors (0.11)
Find the # of comp Sci majors (0.16)
Multiply them: (0.0176)

So that would mean there is a 1.76% chance of being a student who is both a math and comp Sci major.

Yes?

Your answer is wrong, but the question is impossible to answer. Adding the two together and dividing by the total tells you the probability that a student is a math OR comp sci major, which is clearly not correct. But you don't have enough information to give the probability of being both. There could be zero students with both, or there could be 72.

I suspect that the intent of the question is to calculate the probability as P(A and B) = P(A) x P(B), where P(A) = 72/650 and P(B) = 105/650, but that only works when the two events are independent (like two rolls of a fair die). In this case they aren't independent because a student deciding to have two majors is not a random event. But if you need to give an answer to the problem to get credit that's probably the correct one, just understand that it isn't right outside of your homework.

Edit: the second part is what your edit is doing.

So my edit (the 1.76%) is "correct" but only possibly for book purposes?


Automatically Appended Next Post:
And yeah, I kind of got the feeling that there needed to be more information given in the question too in order to properly solve the problem.

 TheMeanDM wrote:
So my edit (the 1.76%) is "correct" but only possibly for book purposes?

Probably, unless there's more to the question that you didn't include. It's wrong, but it seems to be the answer they're looking for.

When in doubt...42. The Universe will abide.

 Peregrine wrote:
Your answer is wrong, but the question is impossible to answer. Adding the two together and dividing by the total tells you the probability that a student is a math OR comp sci major, which is clearly not correct.

That's only true if you know the events are mutually exclusive (ie. you know that there's no overlap between comp sci and math students).

It sound like the sort of question they ask you while they're still trying to teach you Venn diagrams and logic tables If they don't tell you the two events are independent, because essentially what you're trying to calculate is the "intersection" {P(A ∩ B)} and to do that you need to use the "rule of multiplication" as outlined here...

http://stattrek.com/probability/probability-rules.aspx

So you're trying to figure out...

P(A ∩ B) = P(A) P(B|A)

But you've only been given P(A) and P(B), P(B|A) is the probability B can occur if A has already occured, if the events are independent then P(B|A) = P(B).

So in answer to the question, if it was worded exactly how you said it was, I'd say the correct answer is "Assuming the events are independent, the probability is blah blah". If you have to make an assumption to answer a question then you'd include that in your answer. Of course, typically, test questions aren't typically asked in such a way that they require an additional assumption because it makes it annoying to mark if some students who get the correct answer don't list their assumptions (so you usually make part of the question "list your assumptions" if you know extra assumptions are required).

EDIT: Also your 1.76% has a rounding error, it should be 1.79%. Always round off at the end, not mid calculation!

The real answer is to burn the campus down in a cleansing fire thus freeing all the students and creating a wandering band of minstrels and miscreants that will get into all sorts of shenanigans. Then you will be free to take a tricycle into space and seek out inner peace.

Share this answer as I'm sure an instructor will be impressed with your knowledge and automatically pass you I bet. That is when you flip your desk over in a show of dominance.

 TheMeanDM wrote:
I need a bit of help with my statistics question....because what I am thinking is correct I am thinking may not be correct...lol.

So you have 650 students.
72 of them are math majors
105 are comp Sci majors

What is the probability of a student being *both* a math and comp Sci major?

For each of the 650 students, picking one at random, there is a 36/325 chance they are a maths major.

For each of the 650 students, picking one at random, there is a 21/130 chance they are comp sci.

If being a comp sci and/or maths major are independent of each other, then the probability of both events occurring is the multiplication of their respective probabilities, in this case (36/325)*(21/130), which gives you 0.01789. So there is a 1.789% chance that if you pick one person of the group, they will be both a maths and comp sci major. So your edited method is correct for this question but you should avoid rounding any of your numbers until you get to the final answer. Keep the probabilities in fraction form until you need to get the final answer out.

 A Town Called Malus wrote:

 TheMeanDM wrote:
I need a bit of help with my statistics question....because what I am thinking is correct I am thinking may not be correct...lol.

So you have 650 students.
72 of them are math majors
105 are comp Sci majors

What is the probability of a student being *both* a math and comp Sci major?

For each of the 650 students, picking one at random, there is a 36/325 chance they are a maths major.

For each of the 650 students, picking one at random, there is a 21/130 chance they are comp sci.

If being a comp sci and/or maths major are independent of each other, then the probability of both events occurring is the multiplication of their respective probabilities, in this case (36/325)*(21/130), which gives you 0.01789. So there is a 1.789% chance that if you pick one person of the group, they will be both a maths and comp sci major. So your edited method is correct for this question but you should avoid rounding any of your numbers until you get to the final answer. Keep the probabilities in fraction form until you need to get the final answer out.

I believe this the correct method.

However, I have a strong feeling that being a Math and Computer Science Major are not in-fact independent. Math and Computer Science are pretty heavily related fields, so I would actually be suspicious of there not being some relationship. Which would mean your assumptions incorrect. But I suspect for the purposes of them asking this question, they're wanting you to assume they are independent.

 Grey Templar wrote:

 A Town Called Malus wrote:

 TheMeanDM wrote:
I need a bit of help with my statistics question....because what I am thinking is correct I am thinking may not be correct...lol.

So you have 650 students.
72 of them are math majors
105 are comp Sci majors

What is the probability of a student being *both* a math and comp Sci major?

For each of the 650 students, picking one at random, there is a 36/325 chance they are a maths major.

For each of the 650 students, picking one at random, there is a 21/130 chance they are comp sci.

If being a comp sci and/or maths major are independent of each other, then the probability of both events occurring is the multiplication of their respective probabilities, in this case (36/325)*(21/130), which gives you 0.01789. So there is a 1.789% chance that if you pick one person of the group, they will be both a maths and comp sci major. So your edited method is correct for this question but you should avoid rounding any of your numbers until you get to the final answer. Keep the probabilities in fraction form until you need to get the final answer out.

I believe this the correct method.

However, I have a strong feeling that being a Math and Computer Science Major are not in-fact independent. Math and Computer Science are pretty heavily related fields, so I would actually be suspicious of there not being some relationship. Which would mean your assumptions incorrect. But I suspect for the purposes of them asking this question, they're wanting you to assume they are independent.

I agree that there would likely be a connection between the two due to the interconnected nature of the fields but, without it being stated in the question or any indication of what the overlap would be, I would be hesitant to put that into an answer. This is quite a basic question, likely as an intro to stats and the probabilities of multiple outcomes occurring (which any mathhammer fan of this site will be familiar with, albeit maybe not know that that is what they are doing ), so it is possible that they just haven't covered dependent probabilities yet.