Answer
The magnitude of the average force exerted on the water by the blade is $960N$
Work Step by Step
According to the impulse-momentum theorem, $$\sum\vec{\overline F}\Delta t=m\vec{v}_f-m\vec{v}_0$$ $$\sum\vec{\overline F}=\frac{m}{\Delta t}(\vec{v}_f-\vec{v}_0)$$
The stream's initial velocity is $\vec{v_0}=+16m/s$, its final velocity is $\vec{v_f}=-16m/s$, and the mass of water per second is $\frac{m}{\Delta t}=30kg/s$
The magnitude of the average force exerted on the water by the blade is $${\overline F}=|30(-16-16)|=960N$$
