And they also don't consist of 99% water. That is why they called them "purely mathematical potatoes" and they could've chosen any type of fruit or vegetable. Heck, I'm just waiting for a car analogy now!

Brake fluid anyone?

But that 99% water simplification is needed for the purpose of the exercise.

The left out overnight is basically an absurd statement that is intended to confuse and not really related to the actual question.

The statement could be “left in a dry environment until” or simply “left to dry a few weeks”

You definitely could write that but it wouldn't change anything and you could make the same "argument" you are making now. "The left to dry a few weeks is basically an absurd statement that is intended to confuse" and it still wouldn't be true. It's not intended to confuse at all. It's intended to get the point across that you let this imaginary thing dry from 99% to 98%. They could've said "sponge" and let it out to dry any number of minutes. The point isn't to make a 100% accurate example of the drying properties of any actual 'thing'. They just needed something that people intuitively know "has water content" and that "can dry".

Wow by the downvotes I learned people react extremely negatively to any critique of math word problems. Spherical cows and 99% water potatoes are fine, those over simplifications are required for the analysis.

Saying Leaving the potatoes out overnight to imply they halved in mass, sounds as reasonable as “the potatoes were in the ground for 12 hours and then doubled in amount, what percent of water are they now?” It’s so gratuitous and requires ignoring all other laws of physics, while the goal of the spherical cow type simplification is to only ignore a few key challenging ones.

You probably won’t be thrilled by the spherical cows in the nearby pasture then.

Do spherical cows dream of mathematical potatoes?

Spherical cows, naturally, dream of spherical potatoes in a vacuum whilst grazing next to spherical chickens.

Honest question to know how others think.

It doesn't really matter, does it? The rate of evaporation is irrelevant to the problem. Mr. Potato could have waited a year, or dried them on the Uyuni salt plains.

Why do you care? Would this distraction affect your ability to solve the problem?