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"I have a pile of watermelons in a warehouse that weigh 20kg. I have determined that 99% of the weight of the watermelons is water. After a week sitting in the warehouse, some of the water has evaporated so that the watermelons are now 98% water. How much do they now weigh?"
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Well, to begin with there is a total weight of 20kg - 200g of "dry melon" and 99 times that of water, or 19,800g. If after a week the water is down to 98%, the percentage that is dry melon must have risen to 2%. This means there is now 49 times as much water as melon, 9800g, and the total weight is 10kg.