A 6cm pipe narrows to 4cm. When water flows at a certain rate the pressures are 32kPa and 24kPa. What is volume rate of flow?

Use Bernoulli's equation:


P_{Static} + \frac 1 2 \rho V^2 + \rho g h = Constant


So here:


P_1 + \frac 1 2 \rho V_1^2 = P_2 + \frac 1 2 \rho V_2 ^2


and also from constant volume flow:


V_1 * A_1 = V_2 * A_2


That gives you enough to complete the problem.