3-12 The water in a tank is pressurized by air, and the pressure is measured by a multifluid manometer as shown in Fig. P3-12. Determine the gage pressure of air in the tank if h1 = 0.4 m, h2 = 0.6 m, and h = 0.8 m. Take the densities of water, oil, and mercury to be 1000 kg/m3, 850 kg/m3, and 13,600 kg/m3, respectively.