A river flows to the South at 3 km/hour. The city of Townsville sits on the river's bank. Joe's house is 15 km North of the city, also on the river's bank. Joe can paddle his kayak so that he moves at a speed 5 km/hour relative to the water.
At the location of Joe's house, the river is W = 600 m wide. Joe's girlfriend, Anne, lives in a house which is on the East bank of the river, directly across the river from Joe's house.
Joe wants to paddle his kayak so that he will travel in a straight line to Anne's house. He discovers that if he points his kayak straight to the East, he ends up far below Anne's house when he reaches the East bank.