help in probability

[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?

[Kobe]

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]

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]

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]

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]

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 &lt;---- that is the probability


i think that is right.

Are you high? lol

[jazzyb]

Originally posted by gretz


Are you high? lol

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

[TYMSMNY]

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]

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]

After careful analysis, jazzyb is correct.

[gretz]

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]

^^in addition, percents are not probabilities.

[Disoblige]

Originally posted by Guillermo
^^in addition, percents are not probabilities.
It can be represented as a percentage though.

[TYMSMNY]

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. :D

[Guillermo]

Originally posted by Disoblige

It can be represented as a percentage though.

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

[pf0sh0]

Originally posted by Guillermo


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

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

[davidI]

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]

:facepalm: .... :rofl:


Originally posted by Guillermo


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

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

[jazzyb]

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

[TYMSMNY]

Originally posted by Hakkola
:facepalm: .... :rofl:



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

Should keep it to minimum two decimal places.;)

[Guillermo]

probabilities and percents are two different things, and both have their own mathematical definition. By definition, probabilities have to be equal to or between 0 and 1. On the other hand, percents can be negative or can be larger than 100%. Of course, you can convert a probability to a percent, but a probability is not a percent and vice versa.

Just saying.

[pf0sh0]

jazzyb I have tons of practices tests and quizzes on probability just PM me

[gretz]

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

If you understood my answer, you would realize that I expressed "100%" as a character, not a value... lol

[themack89]

Why can't percentage be expressed as a probability?

% = out of 100

25% = 25 out of 100

You have a 25% chance to get an erection when looking at naked men.

You have a 25 in 100 chance to get an erection when looking at naked men.

For every 100 naked men you look at, the likelihood of getting an erection is from 25 of the naked men.

[Disoblige]

Originally posted by themack89
Why can't percentage be expressed as a probability?

lol I wonder what 'Guillermo' is thinking right now.. :rofl:

Probably :banghead:

[rinny]

Without reading anyone else's reply...

I took this course last quarter...

Here is my answer...

Draw a tree diagram:


---10C3 (BOY ON FIRST)---9C2 (BOY ON SECOND)
------------------------------9C7 (GIRL ON SECOND)

---10C7 (GIRL ON FIRST)---9C3 (BOY ON SECOND)
----------------------------9C6 (GIRL ON SECOND)

SO! Add up the choices of the branches that you WANT. Which means, you are only concerned about the situations where you have a girl, chosen second, as the first choice does not matter.

10C3*9C7 + 10C7*9C6 = 4320 + 4320 = 8640 choices



But now that I see you needed probabilities, I agree with jazzyB.