Answer:
[tex]\frac{851}{1000}[/tex]
Step-by-step explanation:
First, we can simply multiply that number by 1000, and divide again by 1000 to get a base fraction:
[tex].851\\\\= \frac{1000}{1000} \times .851\\\\= \frac{1000 \times .851}{1000}\\\\= \frac{851}{1000}[/tex]
851 is a secondary prime, having only two factors, both of which are prime. Those factors are 23 and 37, neither of which is a factor of 1000, so this is already in simplest form.