1) Given those data, let's write:
Hassan 5/6 of a mile: 5/6m
Eva 4/7 of mile = 4/7m
2) Since we want to know the difference, then we must subtract 4/7 from 5/6
Let's call by d, the distance. Then we can write
5/6 -4/7 = d
Rearranging it, adding 4/7 to both sides, and subtracting d from both sides, we have:
5/6 -d = 4/7
And since 5/6 -d = 4/7 will yield the same as 5/6-4/7 = d
3) Hence, The answer is
5/6 - d=4/7