Question 1198097
<pre>


         1    2    3    4    5    6    7    8    9

  1      2    3    4    5    6    7    8    9    10
  2      3    4    5    6    7    8    9    10   11
  3      4    5    6    7    8    9    10   11   12
  4      5    6    7    8    9    10   11   12   13
  5      6    7    8    9    10   11   12   13   14
  6      7    8    9    10   11   12   13   14   15
  7      8    9    10   11   12   13   14   15   16 
  8      9    10   11   12   13   14   15   16   17
  9      10   11   12   13   14   15   16   17   18


If the numbers are chosen with replacement (such as picking one ball from a jar of nine, each labeled with the digits 1..9, and then replacing the first ball before drawing the 2nd), you get P(sum=16) = 3/81 = 1/27 which is approximately 0.037.

If the numbers are chosen without replacement (such as taking two balls out of a jar of nine balls each labeled with one of the above digits) then there are only two ways (7 & 9, or 9 & 7), out of 9*8=72 outcomes, to get a sum of 16.  In this scenario, P(sum=16) = 2/72 = 1/36 or approximately 0.028.