How many kilograms of water is in a 66-kg person

First, you need to find how much water is in an average person with an average weight. Then, you can find a constant rate of proportionality to calculate how many is in a 66 kg person.

The human body is over 90% water, if I remember correctly.

So, a 66 kg person is made up of a little less than 66 kg of water.