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A solar-heated house stores energy in 5.0 tons of Glauber salt (Na2SO4·10H2O), which melts at 90°F. The heat of fusion of Glauber salt is 104 Btu/lb and the specific heats of the solid and liquid are, respectively, 0.46 Btu/lb·°F and 0.68 Btu/lb·°F. After a week of sunny weather, the storage medium is all liquid at 95°F. Then comes a cloudy period during which the house loses heat at an average of 20.000 Btu/h. (a) How long is it before the temperature of the storage medium drops below 60°F? (b) How much of this time is spent at 90°F?
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