Answer
The mass of calcium carbonate=2.95g
Work Step by Step
The molar mass of Ca=40.708amu
The molar mass of Calcium carbonate=100.0869
The percent composition of calcium in calcium carbonate:
Percent composition of calcium=$\frac{\text{molar mass of calcium}}{\text{molar mass of calcium carbonate}}\times100=\frac{40.708}{100.0869}\times100\approx40.67\%$
Recommended daily dietary allowance of Calcium in grams=$1200mg\times\frac{1g}{1000mg}=1.2g$
Let the mass of calcium carbonate be $x$
Mass of calcium=mass of calcium carbonate$\times$ Percent composition of Calcium
$1.2g=x\times40.67\%$
$x=\frac{1.2g}{40.67\%}\approx2.95g$
The mass of calcium carbonate=2.95g