Answer:
Joe
Explanation:
Jon wrote 13 out of 18
Dividing 13 by 18 we get, decimal form

Jim wrote .7 (repeated .7)
0.777..........
Joe reduced his to 5/6
Dividing 5 by 6 we get, decimal form

These numbers are irrational numbers
0.833......>0.777........>0.722......
So, Joe had the higher grade.