You have 100kg of potatoes, each containing 99\\% water. You leave them to dry until they each contain 98\\% water. You now have only 50kg of potatoes.
Proof:
Let p be the weight of the potatoes
p = 100kgand w the water content
w = \frac{99 \cdot 100kg}{100}We know d the dry content is
d = \frac{1 \cdot 100kg}{100}Thus the ratio between d and w is 1:99 and d + w = p .
At 98\% water, the ratio becomes 2:98 or 1:49 . Thus the new water content w' is
w' = \frac{49 \cdot 100kg}{100}and the new weight is
p' = w' + d = 50kg