Tricky Watermelon Water Weight Calculation

What is the question?

Initially, since 99% of watermelon is water, the weight of water must be 99 pounds and weight of other stuffs should be 1 pound.

After evaporation, 98% of watermelon is water indicates that the there is 2% of other stuff present at the point of time. 

That is, if we assume the weight of other stuff unchanged during evaporation, now 1 pound itself is equal to the 2% of total weight of watermelon.

In other words, the process of evaporation increased percent weight of other stuffs in total weight of watermelon from 1% to 2%.

If W is the total weight of watermelon, then weight of other stuffs (which is 1 pound)

W x (2/100) = 1 pound

W = 50 Pounds.

Out of these 50 pound, 1 pound (2% of total weight) is other stuffs and 49 pounds is water (98% of total weight).

That means, out of 99 pounds of water, 99 - 49 = 50 pounds of water is evaporated in the process.