Division of not so little numbers
Haskell, 87 bytes

(a#b)c|s&lt;-show$div(a*10^c)b,l&lt;-length s-c,(h,t)&lt;-splitAt l s=['0'|l&lt;1]++h++['.'|c&gt;0]++t


Usage example: (13#7)27 -> "1.857142857142857142857142857".

23 bytes to handle the c==0 case and using a leading zero instead of things like .5.

How it works: multiply a with 10^c, divide by b, turn into a string, split where the . must be inserted, join both parts with a . in-between and fix the edge cases.