Potatoes are made up of 99% water and 1% "potato matter." Jack bought 100 pounds of potatoes and left them outside in the sun for a while. When he returned, he discovered that the potatoes had dehydrated and were now only made up of 98% water. How much did the potatoes now weigh?

If 100 pounds of potatoes is 1% potato matter, then that means there is one pound of potato matter. After the potatoes dehydrated somewhat, this one pound accounted for 2% of the total mass (with the remaining 98% being water). One pound is 2% of 50 pounds, so the total mass of the potatoes after sitting out in the sun is 50 pounds.