I think it's interesting that just the percentage stated makes it hard to comprehend intuitively.

That is, restate the question with a different end water percentage and the answer is immediately obvious:

> You have 100 lbs of Martian potatoes, which are 99 percent water by weight. You let them dehydrate until they're 50 percent water. How much do they weigh now?

And, of course, it's pretty easy to get "2 pounds", but your brain is pretty fixed on the numbers all clustered together in the other example.