I need help with this multiple choice question involving the conservation of mom

Question

I need help with this multiple choice question involving the conservation of momentum:

A motorboat moves upstream in a river at a speed of 9 m/s relative to the land. The river flows at a velocity of 1.5 m/s.

The boat is powered by a jet-propulsion unit which takes in water at the bow (front) and discharges it at the stern (back). Measurements in the exit jet show its velocity (relative to the boat) to be 18 m/s.

For a flowrate through the unit of 0.15 m3/s, what is the propulsive force produced?

Explanation / Answer

VELOCITY OF BOAT WRT RIVER=9+1.5=10.5m/s

exit jet velocity relative to the river v=18-10.5=7.5 m/s

propulsive force=dm/dt *v

=(0.15*1000)*7.5

=1125N

1123 N is near answer

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