# probability question

Discussion in 'DAT Discussions' started by msu08, Jun 3, 2008.

1. ### msu08 7+ Year Member

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math isn't my thing, and probability certainly isn't my strong point. I constantly have issues with problems like this:
A fair coin is tossed 6 times. What is the probability that exactly 2 heads will show?

the answer is apparently 15/64. I think I understand the 64 is from 2^6, but that's about all i've got.
any help/suggestions? thank youuuuu!

2. ### vvvv 2+ Year Member

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Use this formular

n
= number of trials
r = number of specific events you wish to
obtain
p = probability that the event will occur
q = probability that the event will not occur
(q = 1 - p, the complement of the event

3. ### happyasaclam88 7+ Year Member

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wow ... that formula is great! thanks so much!

4. ### Dencology 2+ Year Member

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can you explain how you used this formula. where do you put the numbers. and what is C and is n raised to C? how do you do the Cr?
thanks.

n
= number of trials
r = number of specific events you wish to
obtain
p = probability that the event will occur
q = probability that the event will not occur
(q = 1 - p, the complement of the event[/quote]

5. ### arpitpatel86 2+ Year Member

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ya can some explain the formula nad how to use it?

6. ### smile101 2+ Year Member

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Probability isn't my best subject either, so I have been practicing (Oh and Kaplan online and SAT math are the best sources). Here's the formula:
nCr means choose 2 from 6 (thats your combination), now multiply by the probability of the desired event (i.e. obtaining 2 heads, so you have (1/2)^2) and multiply this by the probability of not obatining heads (means 4 tails, so you get (1/2)^4)

15 x (1/2)^2 x (1/2)^4 = 15/64

Try this one: For the next five days, the weather channel forecasts 70% chance of raining. What is the probability that it will rain exactly 2 days?

7. ### msu08 7+ Year Member

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where is the 15 coming from exactly? sorry it's a stupid question but i'm still a little confused

8. ### Danny289 Member 2+ Year Member

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Wait for streetwolf.

9. ### Danny289 Member 2+ Year Member

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Thanks, today was plan study this formula for Genetics problems.Now I can study somethging else.

10. ### bumpski20 2+ Year Member

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the 15 is coming from the n C r. the C doesn't actually equal a number. in this case we would right 6 C 2. this is what 6 C 2 actually looks like in equation form:

n! / [r! x (n-r)!] so for this problem: 6! / [2! x (6-2)!]

this comes out to (6 x 5 x 4 x 3 x 2 x 1) / (2 x 1 x 4 x 3 x 2 x 1)

to avoid calculations it may be easier to do (6 x 5 x 4!) / (2 x 1 x 4!)
now you can cancel out the 4! on the top and bottom and your left with (6 x 5) / (2 x 1) which equals 15.

11. ### ttc23 7+ Year Member

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that equation is great.. but it's also not too hard nor time consuming to count out all the possibilitues..

number corresponding to the order of attempt with a head..
12, 13, 14, 15, 16
23, 24, 25, 26
34, 35, 36
45, 46
56

5+4+3+2+1 = 15

12. ### Streetwolf Ultra Senior Member Dentist 7+ Year Member

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Awesome seats at a Yankee game > SDN

Looks like everything is covered here.

BTW ttc23 try doing that for something like (14 choose 5)

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