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