A metal container full of honey weighs 6 kg, and full of water weighs 3.5 kg. If we know that honey weighs twice as much as water, How much does the container weigh?
If the honey weighs twice as much, changing water for honey is equivalent to putting twice as much water. That is, as it goes from 3.5 to 6 kilograms, we are actually increasing 2.5 kilograms. That means that the original water content of 3.5 kilograms is 2.5 kilograms, so the container weighs exactly one kilo.