You can paddle your canoe seven miles per hour through any placid lake. The stream flows at three miles per hour. The moment you start to paddle up stream a fisherman loses one of his bobbers in the water fourteen miles up stream of you.
How many hours does it take for you and the bobber to meet?
It will take 2 hours to meet.
Method 1: Ignore the speed of the stream, as the bobber will be carried along at three miles per hour as will you. It takes two hours to travel fourteen miles, at a rate of seven miles per hour.
Method 2: As the bobber travels at 3 mph, it will be six miles closer to you in two hours. The distance between you and the bobber becomes 8 miles (14 – 6). In two hours you would have travelled 8 ( (7-3) x 2 ) miles.