On average up to 70 of the human body is water a person weig

On average up to 70% of the human body is water, a person weights 70kg, and everyday he drinks 3kg of water, estimate the residence time of the water in his body?   

Solution

70% of body is water , so total water 70*0.70=49kg
one person take 3 kg water per day
so, assuming old water in the body will be replaces by the new taken water.
so, if 3 kg water is taken today ,will be discharged after all the 49 kg water remove.
so time taken = 49/3 =16.33days