Okay lets start this off by rewriting the decimal number as a fraction with 1 as the denominator.
42.875 = 42.875/1
Now we multiply to remove 3 decimal places. Here you will multiply top and bottom by 10^3 = 1000
42.875/1 • 1000/1000 = 42875/1000
The GCF is 125 so.... look below
42875 / 125 = 343
1000 / 125 = 8
= 343/8
=42 7/8