The surprising answer is 50 kg!
https://en.wikipedia.org/wiki/Potato_paradox
100 kg of potatoes, 99% water, means that there's 99 kg of water, and 1 kg of solids. It's a 1:99 ratio.
If the water decreases to 98%, then the solids account for 2% of the weight. The 2:98 ratio reduces to 1:49. Since the solids still weigh 1 kg, the water must weigh 49 kg.