The midpoint of two numbers on a number line can be calculated by finding the sum of the two given numbers and dividing that sum by 2. The resulting answer will be the midpoint of the given numbers.
In other words we can say that midpoint is the average of the given numbers. The given numbers are -9 and 15. The sum of these two numbers is:
-9 + 15 = 6
Half of 6 is 3, so 3 is the midpoint of -9 and 15 on the number line.
In order to verify you can calculate the difference from 3 to 15 and from 3 to -9, the result will be the same.
Do yo do you have anymore information then this? But what you gave me I think it is $15. There are 15 prizes and this are just the ticket not just paying for the item