# need prob. help fast

• Jul 13th 2007, 04:14 PM
sam48
need prob. help fast
Find the probability of rolling a total of 8 using 3 die.

I want to do this without having to write down every option individually.

Thanks for the help.
• Jul 13th 2007, 04:32 PM
galactus
The total number of combos is $6^{3}=216$

$\left(\sum_{k=1}^{6}x^{k}\right)^{3}=(x+x^{2}+x^{3 }+x^{4}+x^{5}+x^{6})^{3}$

Can you expand this?. Look at the coefficient of the x term whose exponent is 8. That's the number of ways it's possible to sum to 8 with 3 dice.

The expansion will start with x^3 and the first coefficient is 3. So, we have $x^{3}+3x^{4}$, thus far.

Now, to find the next coefficient, multiply the coefficient of the x^4 term by the exponent 4 and divide by one less than the preceding exponent. (3)(4)/2=6

$x^{3}+3x^{4}+6x^{5}$

(6)(5)/3=10:

$x^{3}+3x^{4}+6x^{5}+10x^{6}$

Proceed in that manner a few more times and we get $21x^{8}$

The probability of rolling a sum of 8 with 3 dice is $\frac{21}{216}=\frac{7}{72}$