# Homework Help: Rolling Dice Probability (Urgent)

1. Jan 11, 2005

### KingNothing

A pair of dice is being rolled. The probability for rolling a sum greater than 8 is 10/36.

Is the probability for it taking two rolls to attain a sum greater than 8 just 26/36 times 26/36?

I need to make a table of the number of rolls it takes to obtain a sum greater than 8, and the probability that it will take that many rolls. What equation do I use? Assume n is the number of rolls it takes.

EDIT: I think I figured it out as I was walking away from the comp. Is it (26/36)^(n-1) * (10/36)?

Assuming it is, how do you find the average number of rolls it takes to get that? It looks to be about 2.4. But how do I calculate that? Is it just 36/10 or 3.6?

Last edited: Jan 11, 2005
2. Jan 12, 2005

### cepheid

Staff Emeritus
Umm...but the thing is...I interpret the sentence "the probability that it will take two rolls to attain a sum > 8" to mean that you actually get a sum greater than 8 on the second roll. Otherwise, it would take 3 or more rolls! So why did you multiply by 26/36 the second time?

In general, I am not sure about the strategy of multiplying the probabilities together. Wouldn't you expect the probability of obtaining a sum > 8 to increase with larger n? Yet, if you multiply the probabilities, the product only gets smaller.

Yeah, they are not independent events, because if you consider the events independent and multiply the probabilities together (using your formula)...you are calculating the chances of getting a sum less than eight exactly n-1 times, followed by a sum > 8 the nth time. So that's NOT the way to do it.

I'll have to think about it more. No doubt somebody will explain how to do it before I figure it out.

Last edited: Jan 12, 2005
3. Jan 12, 2005

### vincentchan

when you said you roll two times... how is it differ from rolling 4 dices at the same time??? ..... hope this answer your question....

4. Jan 12, 2005

### KingNothing

It just makes it more complicated to think of rolling four dice and dealing with sums greater than 8.

5. Jan 12, 2005