Let the distance to his office be x, then On monday, speed = x/20 miles per minutes = 3x miles per hour On tuesday, speed = (3x + 15) miles per hour Time = distance / speed (20 - 6)/60 hours = x/(3x + 15) hours 7/30 = x/(3x + 15) 7(3x + 15) = 30x 21x + 105 = 30x 30x - 21x = 105 9x = 105 x = 105/9 = 11.7 miles
Therfore he travels an average of 11.7 miles to work.