6
$\begingroup$

I'm studying for a probability exam and came across this question. I watched the video solution to it but I don't really understand it. I was hoping someone could explain this problem to me. Are there different ways to go about this?

$\endgroup$
37
$\begingroup$

Hint:

The probability that an equal number of tails and heads appear is $\large{{2k \choose k} \frac{1}{2^{2k}}}$

The two remaining outcomes (that there are more heads than tails or more tails than heads) are equally likely.

| cite | improve this answer | |
$\endgroup$
  • 4
    $\begingroup$ It seems likely that the "2k" in the question refers to a general even number $2k$ rather than "2000". $\endgroup$ – Chris Taylor May 21 '14 at 10:16
  • 6
    $\begingroup$ @ChrisTaylor: Well, just replace $000$ with $k$, then. ;-) $\endgroup$ – Ilmari Karonen May 21 '14 at 11:13
  • 1
    $\begingroup$ When the question was first posted, I believe OP used "a coin is flipped 2000 times." Also, holy cow, why did this question / my answer get so much attention? Wouldn't think that this answer would be my highest rated non-CW answer.. $\endgroup$ – MCT May 22 '14 at 3:51
13
$\begingroup$

Hint:

  1. Fair coin $\implies$ Probability of tails occurring more $=$ probability of heads occurring more $= p$, say.

  2. Probability of exactly equal number of heads and tails $=1-2p$. Can you find this one?

| cite | improve this answer | |
$\endgroup$
  • 5
    $\begingroup$ @Knickerless-Noggins What on Earth are you talking about? $\endgroup$ – Did May 21 '14 at 8:47
  • 3
    $\begingroup$ @Knickerless-Noggins In any particular set of $2k$ throws with a fair coin, there are three mutually exclusive and collectively exhaustive possibilities - (1) exactly $k$ heads (and tails) occur (2) more than $k$ heads occur or (3) less than $k$ heads occur. Why do you think possibilities (2) or (3) have zero probability? $\endgroup$ – Macavity May 21 '14 at 8:50
  • 3
    $\begingroup$ @Knickerless-Noggins Think of only one throw. What happens? Does this contradict the probability of equal heads and tails? You may need to brush up on reading or have a chat with someone on this, not much point discussing here... $\endgroup$ – Macavity May 21 '14 at 10:43
  • 2
    $\begingroup$ @Knickerless-Noggins: that the coin is fair just means that both sides have an equal chance every single throw. That doesn't mean that 10 throws will always result in 5/5 at all. Try it out for yourself. $\endgroup$ – RemcoGerlich May 21 '14 at 11:37
  • 3
    $\begingroup$ @Knickerless-Noggins The probability that a fair coin produces as many heads than tails in $2n$ throws actually goes to zero when $n\to\infty$. Already when $n=5$ ($10$ throws), this probability is less than $25\%$. $\endgroup$ – Did May 22 '14 at 7:26

Not the answer you're looking for? Browse other questions tagged or ask your own question.