If four watermelons and three melons weigh 15 kg and three watermelons and four melons weigh 13 kg.
How much does a watermelon weigh?
If three watermelons and four melons weigh 13Kg and four watermelons and three melons weigh 15Kg, by putting it all together we have seven watermelons and seven melons weighing 28Kg. Now, if we make seven parts, we have that each pair of a watermelon and a melon should weigh 4Kg.
Three watermelons and three melons are three pairs like the ones we have achieved before, which in total will weigh 4 + 4 + 4 = 12. That means that the melon that is too much in the initial track weighs 13 - 12 = 1kg, so what each watermelon will weigh 3kg.