Hey!
So the number between needs to be the same number away from 23 and 49. 18 is less than 23 and 49 but we need a number between 23 and 49, so thats a no. same with 72.
Now we are left with 36 and 29.5.
well 29.5 is 6.5 points away from 23 but it is 19.5 points away from 49. So thats a no too because the difference is not the same.
now 36 is 13 away from 23 and also 13 away from 49 and the difference is the same
So the answer is C!
Hope this helps!