Chapter 7 - Section 7.6 - Proportions and Problem Solving with Rational Equations - Exercise Set - Page 541: 70

35 mph

against wind: $1365 = (490-x)*t$ with wind: $1575 = (490+x)*t$ $1365 = (490-x)*t$ $1365/(490-x) = (490-x)*t/(490-x)$ $1365/(490-x) = t$ $1575 = (490+x)*t$ $1575/(490+x) = (490+x)*t/(490+x)$ $1575/(490+x) = t$ $1365/(490-x) = 1575/(490+x)$ $1575* (490-x) = 1365 *(490+x)$ $771750 -1575x = 668850+1365x$ $102900 = 2940x$ $102900/2940 = 2940x/2940$ $x=35$