Question 801366
A car travels 400 miles on level terrain in the same amount of time it travels 160 miles on mountainous terrain. If the rate of the car is 30 miles per hour less in the mountains than on level ground, find its rate in the mountains.

let the speed on level ground be x
speed in mountains = x-30

t= d/r

time in mountain = time in level terrain

400/x =160/(x-30)

400(x-30) =160x

400x-12000 = 160x
400x-160x=12000

240x=12000
/240
x=12000/240

x=50

speed on level terrain = 50 
speed in mountain = 50-30 =20

speed in mountains = 20 mph