Let r be the usual speed. Then you have this "time equation"- = 0.5 of an hour. 280*(r+5) - 280*r = r*(r+5) r^2 + 5r - 1400 = 0 (r+40)*(r-35) = 0 The only meaningful is the positive root r = 35 miles per hour. ANSWER Check. - = 4 - 3.5 = 0.5 of an hour. ! Correct !