So to get the answer you need to make a number line from 13 to 37 and you cross off:
13 and 37
14 and 36
15 and 35
16 and 34
17 and 33
18 and 32
19 and 31
20 and 30
21 and 29
22 and 28
23 and 27
24 and 26
and the last number left is the number that's half way between 13 and 37,
so the answer is 25
