Two substances of densities $\rho_1$ and $ \rho_2$ are mixed in equal volume and their relative density is 4. When they are mixed in equal masses, relative density is 3. The values of $ \rho_1$ and $\rho_2$ respectively are

- Density of a mixture depends on the densities of the components and the proportions in which they are mixed (by mass or by volume). - Average density when mixed by volume is the arithmetic mean of the densities. - Average density when mixed by mass is the harmonic mean of the densities, weighted by mass proportion (or a form related to it). The formula derived $\frac{2 \rho_1 \rho_2} {\rho_1 + \rho_2}$ is 2 times the harmonic mean of $\rho_1$ and $\rho_2$.