When we write a fraction by hand we normally have the first number slightly higher off the baseline and the second number slightly below, with a diagonal line between. What I mean is the numerator is above and the denominator is below the dividing line. When we type 1/2 in Word and follow it with a space it is automatically turned into a fraction that looks like this ½. But when we type a non-standard fraction, Word does not treat it the same way, so it stays like this: 3/5. If you require more complex fractions such as 4/15, this is not the answer. The fractions you will find in this feature are the following. 
If you require a fraction such as 3/16 then may I suggest that you read my tip on How to create a customised fraction in Word, to create a more complex fraction that you require.
How can I insert any of these fractions? Well, should you try using the [Insert, Symbol] tool, you’d be heading in the right direction. But it is a little more complicated to find what you are looking for until someone tells you.
The Symbol tool
You can locate the symbol tool in the Insert Tab, all the way on the righthand side. 
- Select the dropdown arrow to reveal a host of previously used symbols and ones you might like to insert at some time.
- Select [More Symbols…] at the bottom of this list.
You are presented with this dialog box that holds ALL symbols. 
At the top right of this dialog box, you will find the [Subset:] This is set to [Basic Latin].
- Change this setting to [Number Forms]. (This is some way down the non-alphabetic list).
- Select the fraction you need and press the [Insert button].
Voila! You have your fraction looking like it should.
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If you need any other fractions, then please read this article I wrote a while back. How to create a customized fraction in Word.
