In this case, we'll have to carry out several steps to find the solution.
Step 01:
Data
Walk-a-thon
total time = 4 days
total distance = 52 miles
equation distance per day = ?
Step 02:
x = distance per day

x = 52 / 4
x = 13 miles / day
The answer is:
The equation is x = 52 / 4, he walked 13 miles per day.