Re: Probability question - honest, not homework!


<djarvinen@xxxxxxxxx> wrote in message
news:1141537892.942400.241600@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Well, as usual, my math skills have failed again, and I can never seem
to get the same answer twice to a question which I am sure is trivial
to most of the readers here.

So I'm rolling a single 6-sided die.

How many times to I have to roll it to give myself a 50% chance of
getting four 1's in a roll?

Honest, not homework! It's just that too many of little grey cells
haved died from a mispent middle-age.


An exact answer to this would be a fair amount of work.

Here is a slightly simpler question, which is far easier.

We roll a dice four times. If they are all 1's, the game is over.
If not, we roll the dice four times again. If they are all 1's the game is
over.
If not, we roll the dice ...

In other words, your question accepts 1231111654221111 as winning on the 7th
move.
In my question 1231 1116 5422 1111 wins on the 16th move, as the four 1's
near the start are in different froups of 4.

I mention this because my question has got an easy, exact answer, whereas
your question is considerably trickier, though a good approximation can be
given quite easily.


.

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