help in probability

**bowsth** · 10-21-2010, 02:06 AM

Suppose a groupe of 3 boys and 7 girls are playing together. The teacher needs to select 2 students. The first student can be a boy or girl, but the second student must be a girl. What is the probability that the second person picked is a girl?

**Kobe** · 10-21-2010, 03:00 AM

Damn i Remeber doing this

it was something like 10C7 on the stats calculator, If I had my calculator I would check it out

Or something like that

**gretz** · 10-21-2010, 07:38 AM

Originally posted by bowsth
Suppose a groupe of 3 boys and 7 girls are playing together.

The teacher needs to select 2 students.

The first student can be a boy or girl,

the second student must be a girl.

What is the probability that the second person picked is a girl?

Since the second student must be a girl, the probability of a girl being picked second is 100%

**davidI** · 10-21-2010, 07:45 AM

Originally posted by gretz

Since the second student must be a girl, the probability of a girl being picked second is 100%

Haha, totally one of those greasy LSAT, GMAT, MCAT type questions.

**jazzyb** · 10-21-2010, 01:37 PM

there are two possible ways to a choose a Girl (G) or second:

a)1st choose boy , then choose girl

b)1st choose girl, then choose girl

for case a: BG = 3/10 * 7/9 = 21/90

b: GG = 7/10 * 6/9 = 42/90

a+b = 7/10 <---- that is the probability

i think that is right.

**gretz** · 10-21-2010, 01:46 PM

Originally posted by jazzyb
there are to possible ways to a choose a Girl (G) or second:

a)1st choose boy , then choose girl

b)1st choose girl, then choose girl

for case a: BG = 3/10 * 7/9 = 21/90

b: GG = 7/10 * 6/9 = 42/90

a+b = 7/10 <---- that is the probability

i think that is right.

Are you high? lol

**jazzyb** · 10-21-2010, 01:50 PM

Originally posted by gretz

Are you high? lol

no, want to show your math? its not possible to have 100%.

**TYMSMNY** · 10-21-2010, 01:53 PM

Originally posted by bowsth
Suppose a groupe of 3 boys and 7 girls are playing together. The teacher needs to select 2 students. The first student can be a boy or girl, but the second student must be a girl. What is the probability that the second person picked is a girl?

10C1 x 7C1 but there is a restriction, if the one person picked is a girl... then there is something you need to do. Can't remember right now.. anyone else complete it?

**jazzyb** · 10-21-2010, 01:58 PM

Originally posted by TYMSMNY

10C1 x 7C1 but there is a restriction, if the one person picked is a girl... then there is something you need to do. Can't remember right now.. anyone else complete it?

Look about 3 posts above you, i'm pretty sure i got it.

**Konj** · 10-21-2010, 02:07 PM

After careful analysis, jazzyb is correct.

**gretz** · 10-21-2010, 02:12 PM

Originally posted by jazzyb

no, want to show your math? its not possible to have 100%.

lol... it was a joke howdy...

Here is my math...

Student picked 2nd is a girl= true, therefore girlpicked2nd = 1

If girlpicked2nd = 1, then probability = $(100%)
else, probability = n
endif

Enter
girlpicked2nd = 1 Enter

100%

Its been a while lol...

**Guillermo** · 10-21-2010, 05:18 PM

^^in addition, percents are not probabilities.

**Disoblige** · 10-21-2010, 05:53 PM

Originally posted by Guillermo
^^in addition, percents are not probabilities.

It can be represented as a percentage though.

**TYMSMNY** · 10-21-2010, 10:08 PM

Originally posted by jazzyb

Look about 3 posts above you, i'm pretty sure i got it.

LOL, right you are. I didn't have a calc in front of me to verify.

**Guillermo** · 10-21-2010, 10:10 PM

Originally posted by Disoblige

It can be represented as a percentage though.

But then it's not a probability, it's a percent.

**pf0sh0** · 10-21-2010, 10:15 PM

Originally posted by Guillermo

But then it's not a probability, it's a percent.

pretty sure a probability can be represented as a percent..

**davidI** · 10-21-2010, 10:38 PM

From what I recall, probabilities can be shown in fractions, ratios, percentages or decimals.

1/3, 1:3, 33.33% or 0.333 are all the same thing.

**Hakkola** · 10-21-2010, 10:56 PM

....

Originally posted by Guillermo

But then it's not a probability, it's a percent.

There's a 67% chance that it's not a probability if expressed in a percentage.

**jazzyb** · 10-22-2010, 08:02 AM

anymore questions of the sort, good practice, i'm writing the gmat in 2-3months.

**TYMSMNY** · 10-22-2010, 08:31 AM

Originally posted by Hakkola
....

There's a 67% chance that it's not a probability if expressed in a percentage.

Should keep it to minimum two decimal places.

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