The weight of the watermelons is reduced by half. Here is the solution:
1,000 kg: 99 % = 990 kg, which refers to the total water volume of the truck load. Thus, the mass is only 10 kg and stays the same during the trip.
At the destination, the mass of 10 kg amounts to 2 % of the total weight. This means that 5 kg equals 1 % and 100 % equals 500 kg.