Question

A police boat is chasing a pirate boat along a straight river by moving in the same direction as the
stream. The speed of the river stream is 5 miles per hour, the speed of the pirate boat relative to
the river is 50 miles per hour, and the police boat is 10 miles per hour slower than the pirate boat.
Currently the pirates are ahead of the police, and horizontally throw a brick at the police boat at a
speed 20 miles per hour relative to their boat. What is the horizontal velocity of the brick relative
to the police boat and to the river bank?
To get full mark, you need to state what