I haven't looked, but I get 50kg.
If we assume that as bought the potatoes were 1% 'potato mass' (i.e. 1kg) and 99% water (i.e. 99kg), and the process of leaving them outside dehydrates them such that no 'potato mass' is lost, i.e. remains at 1kg, then that now forms 2% of the mass, hence the total mass of dehydrated potato is now 1kg /0.02 = 50kg. Thats 1kg of potato and 49kg of water.
