You could pick a number for the total amount of desks, but that will require you to find a good number that is compatible with both 1/2 and 2/3. For this question that is not a big deal, as the previous posts have demonstrated, but if you want a consistently repeatable and efficient strategy, I recommend working with the fractions given and understanding what the question is saying. Especially since the answer choices are fractions, not whole numbers.
If 1/2 of the desks are filled with fifth graders, then there is only 1/2 of the desks available to sixth graders. Since sixth graders fill 2/3 of the remainder (“the remainder” referring to 1/2 of the seats), sixth graders fill 2/3 of 1/2 of the seats. To calculate that fraction, multiply the two fractions together: 1/2*2/3 = 1/3. Sixth graders fill 1/3 of the seats.
BUT! The question is not asking for the proportion of total seats occupied by sixth graders! It is asking for the fraction of sixth graders IN THE DESKS IN USE. meaning we don’t care about the total number of seats anymore, we only care about the seats being used. If 1/2 (aka 3/6) are being used by fifth graders and 1/3 (aka 2/6) are being used by sixth graders, that means 5/6 are being used, and that is the number we need to compare to. 5 units (sixths) are being used, and the sixth graders are occupying 2 of those units, therefore the proportion of used seats occupied by sixth graders is 2/5.
If you grasp the logic quickly, then the math looks like this: (fraction of seats occupied by sixth graders) / (fraction of seats occupied) = (1/2*2/3) / (1/2 + 1/3) = (2/6)/(5/6) = 2/5
Answer D