$10$ A nitrogen gas bubbling flow is used to strip dissolved oxygen gas out of a water tank filled with $20m^3$ water. The total area of the bubbles in the water is $100m^2$ and the mass transfer coefficient of dissolved oxygen on at the bubbles is ${10}^{-5}\frac{m}{s}$ based on the dissolved oxygen concentration. Initially, there is $10\frac{g}{m^3}$ of oxygen in the water. How long does it take to decrease the oxygen concentration of the water to $1\frac{g}{m^3}?$ Give the answer expressed in seconds.

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$9$ A hot ceramic sheet at $100C$ with thickness $1cm$ is cooled from one side by spraying cold water on the surface. The back side of the sheet is insulated. The sprayed surface cools to $20C$ and stays at that temperature. How long does it take for the insulated surface to $cl$ to $30C?$ Assume a thermal conductivity of $1\frac{W}{m}K,$ a heat capacity of $800\frac{J}{k}gK,$ and a density of $1500k\frac{g}{m^3}.$

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