The three pieces of data are given.
1/4 miles in 2/3 min
1/8 mile in 1/3 min
3/8 mile in 1 min
Each of them can be though of as a division or a fraction.
If you multiply the numerator and denominator of a fraction, you get an equivalent fraction.
Start with 1/4 mile in 2/3 min.
Divide both the distance and time by 2.
You get 1/8 mile in 1/3 min. This is the second piece of data, so the first and second pieces of data are proportional.
Now start with the second piece of data:
1/8 mile in 1/3 min, and multiply both by 3.
You get 3/8 mile in 1 minute. This is the third piece of data.
That means all pieces of data are proportional.