Voici le hint:
>.....@x mph.......>7/10 hours<..........@x+75 mph...........<
7/10(x + x+75) = 350
Two planes leave Pittsburgh and Philadelphia at the same time, each going to the other city. One plane flies 75 miles per hour faster than the other. Find the ground speed of each plane if the cities are 350 miles apart and the planes pass one another after 42 minutes of flying time.