The Potatoes - Answer

If 99% of the potatoes' weight is made up of water, then the 100-lb sack contains 99 lbs of water and 1 lb of actual "potato stuff."

If just water evaporates, the amount of "potato stuff" remains constant at one pound.

At the end, the sack contains 98% water; therefore, the one pound of "potato stuff" is now 2% of the weight of the potatoes in the sack.

One pound is 2% of 50 pounds, so at the end the sack weighs 50 pounds.

So 50 pounds of water evaporated! More than you would have guessed?

[If you read the hint, the equivalent problem has an equivalent solution. At the end, there is one woman left in the room, and she represents 2% of the people there. So there are 50 people remaining in the room. Therefore, 50 men left.]