A Venturi meter in a water pipe is 16 cm across at the wide part and 8.0 cm at the throat…

Step 1. Pressure drop. $12\,600\ \text{kg/m}^3 \times 9.8\ \text{m/s}^2 \times 0.030\ \text{m} = 3704\ \text{Pa}$.

Step 2. Speed. $3704\ \text{Pa} = \tfrac12 \times 1000\ \text{kg/m}^3 \times (16 - 1)v_1^2$, so $v_1^2 = 0.494\ \text{m}^2/\text{s}^2$ and $v_1 = 0.703\ \text{m/s}$.

Step 3. Flow rate. $\pi(0.080\ \text{m})^2 \times 0.703\ \text{m/s} = 1.41 \times 10^{-2}\ \text{m}^3/\text{s}$.