We know that 1 gram is equal to $10^{-3}$ kilograms and 1 cubic centimeter is equal to $10^{-6}$ cubic meters. Therefore, the density of the table-tennis ball in kilograms per cubic meter is $0.0840 \times 10^{3} \times 10^{6} = 84 \, \mathrm{kg/m^{3}}$.
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