A boat that moves with a constant velocity of 12 m/s is moving on a river with a constant current of 4 m/s. What is the resultant velocity of the boat if:
a) The boat is traveling to the north and the current is to the south?
b) The boat is traveling to the south and the current is to the south?
c) The boat travels to the east, across the current, while the current is to the south.

When a vertical beam of light passes through a transparent medium, the rate at which its intensity i decreases is proportional to i(t), where t represents the thickness of the medium (in feet). in clear seawater, the intensity 3 feet below the surface is 25% of the initial intensity i0 of the incident beam. what is the intensity of the beam "10" feet below the surface? (give your answer in terms of i0. round any constants or coefficients to five decimal places.)