Re: probability



John Doe wrote:
> I just finished a test and I think the exam writers came up with wrong
> answers for 2 questions[please, let me know if I'm right or wrong]:
> If an ordinary coin is tossed 4 times, what is the probablity that all 4
> tosses will be either all heads or all tails?
> They say 1/16
> I think p = 1/8 because 1/16 to get all heads + 1/16 to get all tails = 1/8
> (because they say either all tails or all heads)
> Am I right?


Yes. It's simply asking whats the probability of tossing the same side
3 more times after your first toss = (1/2)^3 = 1/8


> 2nd question:
> what is the probabilty of obtaining either tails or heads in 5 tosses of a
> fair coin?
> They say 1/2
> I think probability = 1, There are only 2 possibilities. Even in 1 toss
> there is a 100% chance of getting either heads or tails.
> If the question were: what's the probability of getting heads in 5 tosses
> I'd do
> 1 - 1/16 = 15/16 [1/16 probability of getting all tails & 15/16 probability
> of getting at least 1 heads in 4 tosses]


The exam writer wrote in ambiguous language, that's all. Likely he is
not native english speaking.

What he probably meant in Question 1 was "select a side - heads or
tails - what is the probability of tossing that side 4 times in a row"
= 1/16.

Question 2 probably meant "Select a side heads or tails, what is the
probability that you will toss that side after 5 tosses." = 1/2.

.

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