Chapter 7 - Section 7.6 - Proportions and Problem Solving with Rational Equations - Exercise Set - Page 541: 69

510 mph

with wind: $2160 = (x+30)*t$ against wind: $1920 = (x-30)*t$ $2160 = (x+30)*t$ $2160/(x+30) = (x+30)*t/(x+30)$ $2160/(x+30) = t$ $1920 = (x-30)*t$ $1920/(x-30) = (x-30)*t/(x-30)$ $1920/(x-30) = t$ $2160/(x+30) =1920/(x-30)$ $1920 *(x+30) = 2160*(x-30)$ $1920x +57600 = 2160x - 64800$ $57600 = 240x -64800$ $122400 = 240x$ $122400/240 = 240x/240$ $x = 510$