How do you write 8.65 as a mixed number?

It really depends on what you're going for, but 8.65 can be translated as '8 and 65/100', or '8 and 13/20' if you simplify.

This is easier to do when the fractions are as simple as 4.3, because you don't have these huge numbers to deal with and the answer just becomes '4 and 3/10.'

But in any case, the answer that you want is '8 and 13/20.'

8 65/100 is the answer