Based on the way that the answer choices are written, there is a minor math "shortcut" built into this question that you can take advantage of to avoid the last couple of calculations.
Bunuel's calculations are spot-on, so I won't rehash any of that here. Instead, I'll jump straight to the shortcut:
Once you've determined that the return trip takes MORE than 2 hours, since the distance is 30 miles, the average speed for the return trip must be LESS than 15 miles/hour. Here's why:
Using the Distance Formula as reference, we know that if the time was EXACTLY 2 hours, the rate would be 15 miles/hour...
Distance = (Rate)(Time)
30 mi = (15 mph)(2hours)
By increasing the Time, the rate will be decreased (that's the only way that the distance will continue to = 30).
There's only one answer that fits this pattern (and we don't even need to calculate it).
GMAT assassins aren't born, they're made,
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