The hidden questions in this question is basically what Suzy and Gretchen run or walk in a week. You're trying to find the difference, and to be able to do so, you need calculations. You need to multiply each person's rate per day by 7 to find their week rate.
Let's solve.
1.25×7
8.75
This is the distance Suzy runs in a week..
3×7
21
This is the distance Gretchen walks in a week.
Let's subtract.
21-8.75
12.25
So Gretchen covers 12.25 miles more than Suzy.