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# what is probability that 30th day of a specific month is

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what is probability that 30th day of a specific month is [#permalink]

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01 Jan 2007, 11:03
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

what is probability that 30th day of a specific month is friday?
1. 1/8
2. 1/31
3. 1/365
4. 1/52
5. 1/7

I do not have the OA!!!
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01 Jan 2007, 11:34
i would guess that like any other specific day... the probability of it being a friday is 1/7
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02 Jan 2007, 18:29
Specific month- so 1/11 months as february only has 29 days at most. Then 1/7. Then I would mult. the two together. However the answer is not there. Hence think a little more. How many 30th's can be a friday in any given year? Must rotate through days 1-7. If january 30 is friday then you will get one more friday on the 30th. So it should be 1/2 * 1/7 for 1/14?

Thats my thought process. Hope I dont see this on test day 1/11/07, a thursday incidentally.

Cheers
Eric
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03 Jan 2007, 13:09
Definitely it depends on how many days the month has -28, 29, 30, 31- and in which day of the week the month started.

For 30-day mths:

Mo ... Su / Mo ... Su / Mo ... Su / Mo ... Su / Mo Tu: 4 Fridays: probab = 4/30

Th ... Wd / Th ... Wd / Th ... Wd / Th ... Wd / Th Fr: 5 Fridays: probab = 5/30

Fr ... Th / Fr ... Th / Fr ... Th / Fr ... Th / Fr Sa: 5 Fridays: probab = 5/30

And so on.
03 Jan 2007, 13:09
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