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Here is a table of all possible outcomes of sums, count those that
are 7 or less.  There are 36 possible outcomes so that will be the
denominator.
       Die_1  1    2    3    4    5    6
Die_2 
  1           2    3    4    5    6    7
  2           3    4    5    6    7    8
  3           4    5    6    7    8    9    
  4           5    6    7    8    9   10
  5           6    7    8    9   10   11
  6           7    8    9   10   11   12
21/36 = 
 or approx. 0.583 (58.3%)