# Thread: Simple probability question I can't figure out- help please

1. ## Simple probability question I can't figure out- help please

Hi, I have a probability question I would appreciate help with. Question: if you flip a coin, and continue to flip it, how many times until you would on average get two heads in a row (in immediate succession)?

I understand that if you were to flip the coins in groups of two, one in four of those groups would be two heads (HH, HT, TT, TH). What Im less sure about is if youre flipping coins one after the other- at what point, after how many flips, would you on average get two heads (the number including the two heads)?

2. ## Re: Simple probability question I can't figure out- help please

The question isn't clear the way you a phrasing it.

Do you mean what is the expected value of the number of tosses it takes to get two heads in a row?

Yes

4. ## Re: Simple probability question I can't figure out- help please Originally Posted by Alexander2013 Yes
$E = \sum \limits_{k=0}^\infty~\left(1-\dfrac 1 2 \right)^k\cdot \dfrac 1 4 \cdot (k+2) = \dfrac 3 2$

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