Hey there! Hello!
If I understand correctly, 10:85 is a ratio. This is basically the same thing as saying that there are 10 parts per 85 parts. So, this can be represented by the fraction 10/85, as well.
Think of it this way: if you have 10 parts per 85 parts, that remains true if you multiply the numbers by 2; 10:85 or 10/85 will be the same as 20:170 or 20/170, and so on.
So, all you have to do is simplify 10/85. The largest common factor that I see is 5, which makes the simplified fraction 2/17.
Hope this helped you out! Feel free to ask me any additional questions if you have any. :-)