This problem is very straightforward but a few algebra and arithmetic tricks can be illustrated here. Firstly, if you ever have a fraction equaling another fraction, you can imagine that any of the 4 values you see can be picked up and moved diagonally across the equal sign. I want to isolate x. Thus, I simply pick up the 8 and move it diagonally up and across the equal sign next to the 3. So x/8 = 3/5 becomes x = 24/5.

Most people will cross multiply and get 5x = 24, then divide 24 by 5 to isolate x. I don't see the point of multiplying by 5 and then dividing by 5 immediately, though.

At any rate, now we have x = 24/5. What's that? The answer choices, as McGraw-Hill likes to do, are extremely far apart so we could probably just estimate with little risk. They're not even in numerical order, which the GRE will always do. But anyway, 24/5 should be extremely easy to do in your head.

Here's the trick whenever you need to divide anything by 5: divide it by 10 instead. This is easy because we simply move the decimal to the left. But the number we've got is now half the size it should be. So just multiply it by 2! In this case 24 divided by 10 becomes 2.4. Multiplying that by 2 we can easily get 4.8. So it's D.