\(n_{CaCO_3}=\dfrac{20}{100}=0,2\left(mol\right)\)
PT: \(CaCO_3\underrightarrow{t^o}CaO+H_2O\)
\(n_{CaO\left(LT\right)}=n_{CaCO_3}=0,2\left(mol\right)\)
\(\Rightarrow m_{CaO\left(LT\right)}=0,2.56=11,2\left(g\right)\)
\(\Rightarrow H=\dfrac{8}{11,2}.100\%\approx71,43\%\)