Iwrote 5 different numbers on five cards. the sum of the numbers is 15. what numbers did i put on the cards?

If they all have to be positive whole numbers, then the only
way you could do it is  1,  2,  3,  4,  and  5 .
(They could be in any order.)

If they don't all have to be positive, or they don't all have to be
whole numbers, then there are an infinite number of ways to do it.

2 ,  4 ,  6 ,  8 ,  and  negative 5

0.2 ,  0.8 ,  1.0 ,  1.4 ,  and 1.6

etc.