# Probability Question

**URL:** <https://boards.straightdope.com/t/probability-question/10663>\
**Category:** Factual Questions\
**Created:** [April 19, 2000, 8:13am UTC](https://boards.straightdope.com/t/probability-question/10663 "2000-04-19T08:13:00Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![AccorDeezeNutz](https://avatars.discourse-cdn.com/v4/letter/a/6a8cbe/32.png) [@AccorDeezeNutz](https://boards.straightdope.com/u/AccorDeezeNutz)\
**Post date:** [April 19, 2000, 8:13am UTC](https://boards.straightdope.com/t/probability-question/10663/1 "2000-04-19T08:13:00Z")

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Say i flipped a regular coin and on the first try I got heads. If i flipped that coin again what are the chances of me getting heads. would it be: 1/2 cause there is 2 sides. 1/4 cause i have a 1/4 chance of getting heads in 2 tries ( 1/2 \* 1/2 = 1/4). or 1/3 (in between)

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**Author:** ![Cabbage](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@Cabbage](https://boards.straightdope.com/u/Cabbage)\
**Post date:** [April 19, 2000, 8:16am UTC](https://boards.straightdope.com/t/probability-question/10663/2 "2000-04-19T08:16:00Z")

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It would be 1/2. Getting heads for the first flip doesn’t change the probability of getting heads on the next flip.

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…ebius sig. This is a moebius sig. This is a mo…  
(sig line courtesy of WallyM7)

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**Author:** ![JonF](https://avatars.discourse-cdn.com/v4/letter/j/cab0a1/32.png) [@JonF](https://boards.straightdope.com/u/JonF)\
**Post date:** [April 19, 2000, 10:45am UTC](https://boards.straightdope.com/t/probability-question/10663/3 "2000-04-19T10:45:00Z")

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In two flips of a coin, there are four possibilities:

HH  
HT  
TH  
TT

However, given that the first flip came up heads, the possibilities reduce to two:

HT  
HH

You can see that of these possibilities, the coin comes up heads on the second flip 50% of the time.

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jrf

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**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\
**Post date:** [April 20, 2000, 5:29am UTC](https://boards.straightdope.com/t/probability-question/10663/4 "2000-04-20T05:29:00Z")

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And, assuming a fair coin:  
if you flip it 999 times, and all 999 times it comes up tails, what is the probability of getting tails on the 1000th flip? 1/2.