an oil refinery is located at point R on the north bank of a straight river that is 2 km wide. a pipeline is to be constructed from the refinery to storage tanks located at point T on the south bank of the river 6 km east of the refinery. the cost of laying pipe is $400,000/km to run pipe over land to a point P on the north bank of the river and then $800,000/km to run pipe under the river to the tanks. where should the point P be located.