What numbers make 13?

Answer 1

There are infinitely many ways.

Think of any number, say x.

calculate y = 13 - x

Then x + y = 13

Since the choice of x was arbitrary, there are infinitely many answers.

That applies to pairs of numbers. You can repeat the process for sums of 3, 4, ... numbers.

Also, you can look at multiplication and other ways of combining numbers (binary operations).

Answer 2

The numbers that make 13 are 1 and 12, 2 and 11, 3 and 10, 4 and 9, 5 and 8, and 6 and 7.