I took a look at your students exercise and it looks like they are printing out the wrong result.
The student is adding the fractions correctly, however, they have made a small mistake in what they print to the screen. The student is using the numerator of the second fraction as the denominator of the first fraction. For example:
If the input to the program is:
numerator1 = 10
denominator1 = 5
numerator2 = 2
denominator2 = 6
Or more simply:
10/5 + 2/6
The student is correctly finding that the sum is 70/30, but when they print this to the screen they are printing:
10 / 2 + 2/6 = 70/30
10 / 5 + 2/6 = 70/30
It's just a small mix up that is causing the grader to fail.