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When rolling a certain unfair six-sided die with faces numbered 1, 2, 3, 4, 5, and 6, the probability of obtaining face $F$ is greater than $1/6$, the probability of obtaining the face opposite face $F$ is less than $1/6$, the probability of obtaining each of the other faces is $1/6$, and the sum of the numbers on each pair of opposite faces is 7. When two such dice are rolled, the probability of obtaining a sum of 7 is 47/288. Given that the probability of obtaining face $F$ is $m/n$, where $m$ and $n$ are relatively prime positive integers, find $m+n$.

Guest Apr 8, 2015

Best Answer 

 #2
avatar+92781 
+5

Hi Mellie,

It is very late and I am not going to get a final answer tonight but I will show you where my thoughts are taking me.

 

I'm going to let 1 and 6 be the biased sides so 2,3,4 and 5 all have a chance of 1/6 of being rolled.

so the chance of throwing a 1,2,3 or 4 is      $${\frac{{\mathtt{4}}}{{\mathtt{6}}}}$$  =  $${\frac{{\mathtt{2}}}{{\mathtt{3}}}}$$

the chance of throwing a 1 is    $${\frac{{\mathtt{m}}}{{\mathtt{n}}}}$$       and the chance of throwing a 6 is      $${\frac{{\mathtt{1}}}{{\mathtt{3}}}}{\mathtt{\,-\,}}{\frac{{\mathtt{m}}}{{\mathtt{n}}}}$$

 

now to roll a 7 these are the possibilities

 

$$\\P(1,6) =\frac{m}{n}\times\frac{n-3m}{3n}=\frac{m(n-3m)}{3n^2}\\\\
P(2,5) = \frac{1}{6}\times\frac{1}{6}=\frac{1}{36}\\\\
P(3,4) = \frac{1}{6}\times\frac{1}{6}=\frac{1}{36}\\\\
P(4,3) = \frac{1}{6}\times\frac{1}{6}=\frac{1}{36}\\\\
P(5,2) = \frac{1}{6}\times\frac{1}{6}=\frac{1}{36}\\\\
P(6,1) = \frac{n-3m}{3n\times}\frac{m}{n}=\frac{m(n-3m)}{3n^2}\\\\
NOW\\\\$$

$$\\\frac{4}{36}+\frac{2m(n-3m)}{3n^2}=\frac{47}{288}\\\\
\frac{32}{288}+\frac{2m(n-3m)}{3n^2}=\frac{47}{288}\\\\
\frac{2m(n-3m)}{3n^2}=\frac{15}{288}\\\\
\frac{m(n-3m)}{n^2}=\frac{15*3}{288*2}\\\\
\frac{m(n-3m)}{n^2}=\frac{5}{64}\\\\
$Now if we are lucky n will equal 8$\\\\
m(8-3m)=5\\\\
8m-3m^2=5\\\\
$Just by sight I can see that m=1 will work.$\\\\
$So one possibility, maybe the only possibility is $ n=8 \;and\; m=1\\\\
m+n=9$$

 

There is probably a much quicker way of doing this.  Plus I have not shown that this is the ONLY solutions. 

Melody  Apr 9, 2015
 #1
avatar+92781 
+5

What is $F$  ?

The sides are numbered 1,2,3,4,5, and 6   So what is F  ?

Melody  Apr 9, 2015
 #2
avatar+92781 
+5
Best Answer

Hi Mellie,

It is very late and I am not going to get a final answer tonight but I will show you where my thoughts are taking me.

 

I'm going to let 1 and 6 be the biased sides so 2,3,4 and 5 all have a chance of 1/6 of being rolled.

so the chance of throwing a 1,2,3 or 4 is      $${\frac{{\mathtt{4}}}{{\mathtt{6}}}}$$  =  $${\frac{{\mathtt{2}}}{{\mathtt{3}}}}$$

the chance of throwing a 1 is    $${\frac{{\mathtt{m}}}{{\mathtt{n}}}}$$       and the chance of throwing a 6 is      $${\frac{{\mathtt{1}}}{{\mathtt{3}}}}{\mathtt{\,-\,}}{\frac{{\mathtt{m}}}{{\mathtt{n}}}}$$

 

now to roll a 7 these are the possibilities

 

$$\\P(1,6) =\frac{m}{n}\times\frac{n-3m}{3n}=\frac{m(n-3m)}{3n^2}\\\\
P(2,5) = \frac{1}{6}\times\frac{1}{6}=\frac{1}{36}\\\\
P(3,4) = \frac{1}{6}\times\frac{1}{6}=\frac{1}{36}\\\\
P(4,3) = \frac{1}{6}\times\frac{1}{6}=\frac{1}{36}\\\\
P(5,2) = \frac{1}{6}\times\frac{1}{6}=\frac{1}{36}\\\\
P(6,1) = \frac{n-3m}{3n\times}\frac{m}{n}=\frac{m(n-3m)}{3n^2}\\\\
NOW\\\\$$

$$\\\frac{4}{36}+\frac{2m(n-3m)}{3n^2}=\frac{47}{288}\\\\
\frac{32}{288}+\frac{2m(n-3m)}{3n^2}=\frac{47}{288}\\\\
\frac{2m(n-3m)}{3n^2}=\frac{15}{288}\\\\
\frac{m(n-3m)}{n^2}=\frac{15*3}{288*2}\\\\
\frac{m(n-3m)}{n^2}=\frac{5}{64}\\\\
$Now if we are lucky n will equal 8$\\\\
m(8-3m)=5\\\\
8m-3m^2=5\\\\
$Just by sight I can see that m=1 will work.$\\\\
$So one possibility, maybe the only possibility is $ n=8 \;and\; m=1\\\\
m+n=9$$

 

There is probably a much quicker way of doing this.  Plus I have not shown that this is the ONLY solutions. 

Melody  Apr 9, 2015

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