<PRE><B>Question 40795</B>
Let r = the trucks rate in the mountains
r + 20 will be the rate on flat land
The semi-truck travels 300 miles through the flatland in the same amount of time that it travels 180 miles through mountains.
Let t = the time for both trips
t = 180/r
t = 300/(r + 20)
180/r = 300/(r + 20)
multiply both sides by r(r+20)
180(r+20) = 300r
180r + 3600 = 300r
subtract 180r from both sides
3600 = 120r
r = 30 the rate in the mountains
r + 20 = 50 the rate on flat land
check
180/r = 300/(r + 20)
180/30 = 300/50
6 = 6
OK </PRE>