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\Delta \, \mathrm{Area} / \Delta \, \mathrm{time}  =\left(706.9 \, \mathrm{ft}^{2}-78.6 \, \mathrm{ft}^{2}\right) / (1 \,  \mathrm{sec})=628.3 \, \mathrm{ft}^{2} / \mathrm{sec}