To find the odds against an event.

1. Form the fraction:

 number of possible ways to fail
-----------------------------------
number of possible ways to succeed

2. Reduce the fraction to lowest terms, retaining the denominator 
1 in case the fraction reduces to a whole number.

3. Express the fraction as a ratio either: 

      (numerator) to (denominator) 

or as

     (numerator) : (denominator)


The ways to fail to roll a prime number is to roll 1, 4, or 6.
That's 3 ways to fail.

The ways the succeed in rolling a prime number is to roll a 2,3, or 5, 
That's 3 ways to succeed.

1.  We form the fraction 

2.  We reduce that to , retaining the 1 denominator.

3.  We express the fraction as a ratio as either 

    1 to 1

    or as  

    1 : 1

Edwin