Question

How many pair of whole numbers have a sum 99

Answer 1

Infinitely many.
45 + 45, 46 + 44, 47 + 43, ... , 98 + 1, 99 + 0, 100 + (-1), 101 + (-2) and so on and on and on. Without an end in sight.

Answer 2

Adding up whole numbers in pairs, we can get (50) fifty successful pairs with "99" as an outcome; it is an easy exercise that we can show in the following illustrative progression as example: (0+99), (1+98), (2+97), (3+96) and so on up until (44+50).