The given information tells us that the halfway between 9 and 16 is 12.5.
We find the number halfway when we sum both numbers and then we divide by 2.
E.g.
9 + 16 = 25
Then 25/2 = 12.5
So, we can deduce the wholes numbers between must be even numbers if we want to get as a result a whole number
We deduce this because their sum is divided by 2.
For example
2 + 2 = 4, then divide by 2, 4/2 = 2
Therefore, the number that is halfway between two whole numbers is going to be a whole number when the two numbers are even numbers.