A piece of brass (Cu and $Z n$ ) weighs $12.9 \mathrm{~g}$ in air. When completely immersed in water, it weighs $11.3 \mathrm{~g}$. The relative densities of $\mathrm{Cu}$ and $\mathrm{Zn}$ are $8.9$ and $7.1$, respectively. Calculate the mass of copper in the alloy (in decigram).