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\Delta \mathrm{A} / \Delta \mathrm{t}=\left(113.1 \, \mathrm{ft}^{2}-78.6 \, \mathrm{ft}^{2}\right) /(.1 \, \mathrm{sec})=345 \, \mathrm{ft}^{2} / \mathrm \, {sec}