When do you try Smart Numbers with a GMAT Rates question? Hardly ever. Why? It just makes things twice as difficult.
This question doesn’t need to take a long time to solve if you do it right, but as you’ll see, it’s not easy either way you cut it.
The hard part, really, is to determine the strategy before you set out and do a bunch of unnecessary Algebra. The key point here is to notice the ugly-ass Algebra you’ll be required to deal with and to move on from there rapidly.
Once you reach the point in the setup where you realize that it’s a Double Plug-In GMAT Rates Question (see here for more details), you’ll then see that you really don’t want to do this Algebra.
At this point, I’d look at the setup you’ve done so far and see which of the answer choices would fit in most easily. In this case, because the time is an hour PLUS 1/2 hour, you want to go for answer choices that are phrased as, say, 2.0, 2.5, 3.0, 3.5, etc.
Obviously, given the answers here, that limits us a bit.