# ada sample 2009-another error on QR?

Discussion in 'DAT Discussions' started by psh3kgtplaya88, Jul 26, 2011.

1. ### psh3kgtplaya88 2+ Year Member

Joined:
Feb 10, 2010
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A container holds 8 red and 4 white balls. two balls are drawn in sequence without replacement from the container. which of the following represents the probability that exactly one of these balls is red?

a. 16/33
b. 32/132
c. 12/32
d. 56/132
e. 32/144

ADA really needs detailed solutions with their correct answers!!!

I solved by doing

1-(probability of picking all white)

1-(4/12-3/11)=13/33

2. ### wenjie1988 2+ Year Member

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Heres how I solved it:

(8/12)(4/11)(2) = 16/33

3. ### herkulease 2+ Year Member

Joined:
Jan 23, 2009
Messages:
898
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Status:
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1st your math is wrong.

1-(4/12 -3/11) = 31/33 not 13/33.

2nd the probability of picking all white is 4/12 * 3/11 not as you have written.

For this problem it is asking for the probability of get EXACTLY 1 red ball. which is the following

(8/12 * 4/11) + (4/12 * 8/11) = 64/132 which is reduced 16/33.

the way you're thinking of determining just the probability of getting all white balls would require the question to be rephrase. instead of reading EXACTLY one red, it would read AT LEAST one red.

be careful these kinds of question. Its how they get you. when you don't pay attention and read to fast under the pressure.

4. OP

### psh3kgtplaya88 2+ Year Member

Joined:
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Messages:
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Status:
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Awesome work guys! thanks for the quick response! I clearly need to brush up my math!

What's the best way to perfect math? People say to do as many questions as you can but, I feel like I'm just memorizing problem types and not understanding the concepts properly...

test is in 3weeks :/

5. ### herkulease 2+ Year Member

Joined:
Jan 23, 2009
Messages:
898
2
Status:
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the best way to tackle math is indeed practice practice practice.

Obviously you'll have to know the various functions and rules but even if you know them for instance the biggest hurdle is being able to breakdown the problem fast enough to solve them. That was my problem and I'm sure a lot of people's problem too.

If you have issues with concepts then 3 weeks time will be tough but not impossible to do a crash course. these weren't available to me but people say chad has some math videos up. Maybe they can help you out in explaining some concepts. Might not help you get a 30 if you're completely lost but they might help you get a decent score.

use google to find them. they are supposedly free.

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