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What is the probability that it will rain tomorrow?

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by gmattesttaker2 » Tue May 06, 2014 10:50 pm
Hello,

Can you please tell me how to solve this:

What is the probability that it will rain tomorrow?

1.The probability that it rains today is 40%.
2.If it rains today, the probability that it will rain tomorrow will increase by 10%.

OA: E

1) In-suff.

2) In-suff.

I was wondering how to solve for E ?

Thanks for your help.

Regards,
Sri
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Source: — Data Sufficiency |
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by Tushar14 » Tue May 06, 2014 11:04 pm
Hello Sri,
According to me it is a natural phenomena and other factors like wind direction & humidity also should be considered which are missing in both the statements. So, both are in-sufficient in this case.
Probability can only be determined in terms of symmetrical outcomes (getting head in a coin toss) or relative frequencies (million tosses for eg).

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Tushar
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by theCodeToGMAT » Wed May 07, 2014 3:59 am
To find: Probability for Rain tomorrow

Statement 1:
Today's Probability = 40%
No info for tomorrow
INSUFFICIENT

Statement 2:
Tomorrow's probability = Todays + 10%
INSUFFICIENT

Combining...
it rains today = 40%
Tomorrow's probability = Todays + 10%
if it rains today, then probability for tomo = 40 + 10 = 50%
if it doesn't rains today, then probability for tomo = 40%
So, we are not sure
INSUFFICIENT

[spoiler]{E}[/spoiler]
Last edited by theCodeToGMAT on Wed May 07, 2014 6:52 pm, edited 1 time in total.
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by Tushar14 » Wed May 07, 2014 6:48 pm
Rahul,
In the last para you mentioned; if it rains today, then probability for tomorrow = 40 - 10 = 30%
Don't you think it should be 40 + 10 = 50% (as statement2 in question says increase of 10% if it rains today)

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Tushar
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by theCodeToGMAT » Wed May 07, 2014 6:51 pm
Tushar14 wrote:Rahul,
In the last para you mentioned; if it rains today, then probability for tomorrow = 40 - 10 = 30%
Don't you think it should be 40 + 10 = 50% (as statement2 in question says increase of 10% if it rains today)

Regards,
Tushar
Yep, I made a typo!

Thanks!
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by Matt@VeritasPrep » Mon May 12, 2014 12:06 pm
Guys, there is a much simpler explanation:

NOBODY SAID THAT THE PROBABILITY OF RAIN TOMORROW STARTS AT 40%!

For all we know the baseline probability of rain tomorrow is 0%, so that if it rains today the probability tomorrow will be exactly 10%. Tomorrow could also have a baseline probability of rain of 90%, so that if it rains today the probability of rain tomorrow is 100% ... and really we could have any other probability in between.

Be careful here!
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