1. **State the problem:** We are given a fluid density of 3 g/mL and a flow rate of 42 mL/s. We need to find the mass flow rate in g/s. 2. **Formula used:** Mass flow rate $\dot{m}$ is given by the product of density $\rho$ and volumetric flow rate $Q$: $$\dot{m} = \rho \times Q$$ 3. **Substitute the given values:** $$\dot{m} = 3\ \text{g/mL} \times 42\ \text{mL/s}$$ 4. **Calculate the mass flow rate:** $$\dot{m} = 126\ \text{g/s}$$ 5. **Interpretation:** The mass flow rate is 126 grams per second, meaning 126 grams of fluid pass through the pipe every second.