#### Answer

Mass % of Mg = 28.827%
Mass % of C = 14.246%
Mass % of O = 56.927%

#### Work Step by Step

Mass percentage of an element =
(mass of that element in the compound × 100)/(molar mass of the compound)
Molar mass of magnesium carbonate ($MgCO_{3}$) = $(1\times24.305g)+(1\times12.011g)+(3\times15.999g)$ = 84.313g
Mass % of Mg = $\frac{(1\times24.305g)\times100}{84.313g}$ = 28.827%
Mass % of C = $\frac{(1\times12.011g)\times100}{84.313g}$ = 14.246%
Mass % of O = $\frac{(3\times15.999g)\times100}{84.313g}$ = 56.927%