What and what equals 19?

Answer 1

There are infinitely many possible answers. The answer depend on the operation:

• Using addition: let A be any number less than or equal to 9.5 and let B = 19-A. Then A + B = A + (19-A) = 19. For each of the infinitely many choices for A, the pair (A, B) is different. So there are infinitely many additive answers.
• Change the sign of B and then every pair A - B is a possible solution.
• Let C be any number in the range [-sqrt(19), sqrt(19)] other than 0, and let D = 19/C. Then C*D = C * 19/C = 19. For each of the infinitely many choices of C, the pair (c, D) is unique. So there are infinitely many multiplicative solutions.
• Replace D by 1/D and you will have infinitely many solutions such that C/D = 19.
• Then there are many other operations.

Answer 2

10 + 9