Your percentage of body fat by weight is an important indicatorof your general physical condition. For people in good health itshould be between 10 and 15%. One way of determining thispercentage is to measure your average body density. This can bedone by weighing yourself underwater when you have completelyexhaled. You can do this in your friendly neighbourhood pool byplacing a kitchen scale on the bottom (one that is not damaged bybeing under water) and submerging yourself onto it when you havedeeply exhaled. (You might want to use a snorkel if you areuncomfortable doing this trick.) You can then calculate your bodydensity from the formula,  

D = W/(W-Ww)

where W is your normal weight and W_w is yourweight underwater. (This formula assumes the density of water isone. If you are mathematically inclined and know a little about thephysics of buoyancy from a high-school physics course you caneasily derive this formula.) If as a result of such anexperiment a 75 kg person is found to weigh 4.2 kg underwater, whatis that person's body density?

Professionals who determine body fat by this method havedeveloped the following mathematical relationship betweenpercentage of body fat f and average body density D:

f = 100 x (4.20/D - 3.81)

What is the percentage of body fat in the person of theprevious part?