The velocity of a plane relative to still air is 135 mr/hr. The pilot wants to fly straught north over the ground. A ground observor tells the pilot by radio that a 70 mi/hr wind is blowing, but forgets to tell her the wind direction. The pilot observes that, in spite of the wind, she can travel 135 miles north over the ground in one hour (i.e., her ground speed is the same as if there was no wind).
What is the direction of the wind relative to the ground?