Ms. J has graded 10 tests already and can grade 8 tests per hour. After h hours, she will have graded an additional 8h tests.
Now, we can set up an equation to find when they have graded the same number of tests:
30 + 10h = 10 + 8h
Now, let's solve for h:
2h = 30 - 10
2h = 20
Divide both sides by 2:
h = 20 / 2
h = 10
So, it will take 10 hours of grading for Mr. S and Ms. J to have graded the same number of tests.