Rewrite as a simplified fraction.
3.5 repeating = ?

As a fraction in simplified form, 3.5 is equal to 7/2 .

But you say you want something that's repeating.  

That's a problem . . . there's no part of 3.5 that's repeating now, and if I just go ahead and write something that does repeat, my invention will change the value, and it won't be equal to 3.5 any more.

Here's one possibility, I guess: 3.500 000 000 000 000 000 000 . . .

The zeros repeat infinitely, and no matter how many (or how few) zeros you write, the value of the number is always 3.5 and it never changes.

If it's the ' 5 ' on the end that you want to see repeating, that's actually easier to work with.  Then we have 3.555 555 555 555 555 555 . . .

THOSE fives really do keep repeating and never end, and that number is equal to (3 and 5/9) or (32/9) .