Fresh grapes contain 80% water by weight and dry grapes contain 20% water. From 60 kg of fresh grapes, the weight of dry grapes obtained is (in kg):

Only water leaves, so the solid pulp is the fixed quantity: 20% of 60 = 12 kg. In the dried fruit that pulp is 80% of the weight, so the dry grapes weigh 12/0.8 = 15 kg. Answering 12 kg forgets that dry grapes still hold 20% water, and 48 kg is the water that was in the fresh fruit.