What would be the decimal fraction between 5.1 and 5.2?

There are an infinite number of them.

Here are a few:

5.11

5.12

5.13

5.14

5.149

5.15

5.15000000001

5.15000000002

5.16

5.17

5.18

5.19

5.199

5.1999

5.19999

etc.