42
19.5 is halfway between 18 and 21
26.5
107
It is: (63+57)/2 = 60
There is no distance between them.
It is: 75
(17 + 63)/2 = 80/2 = 40
There are lots of numbers between 63 and 720. Exactly halfway between them is 391.50, so that may be the answer you are looking for.
Do you mean like what is the number exactly between them. 53+63=116/2=58
37
21