 The challenge to the navigator is to find how long one must sail on the NE return course (the length of side "CD", what is called the "ritorno") and how far one has advanced on the intended course by the time one straightens out (the length of the hypotenuse "AD", or what is called the total "avanzo").This is elementary trigonometry, solving for two sides given one side (70) and two angles (45° and 90°).This is quickly done by applying the law of sines