Brake fluid anyone?
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”
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.
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?