What can go into 13 and 26?

Answer 1

13 is a Prime number, so it's only factor are 13 and 1.

the factors of 26 are 1, 2, 13, and 26.

as you can see, 13 goes in to both numbers. 1 also goes into both numbers, but this solution is considered trivial as 1 goes into every number.

Answer 2

1 and 13.