1/6
1) Since a ratio expresses a relationship between two parts
We can write this about Mallory and the ratio of the cookies she ate. And, as She ate 3 and there are 15 left, so initially there were 18 cookies.
18-3=15
2) So let's consider that, to write our ratio:
3) So Mallory ate 1/6 of the cookies in the jar.