Question 333295
 traveling east. speed= 120 miles per hour faster than the plane traveling west. 

After three hours, the planes were 2,400 miles apart. How fast was each 
let speed of west bound plane be x mph
east bound plane speed will be x-120 mph
..
distance = speed * time
time = 3hours
distance traveled by one plane = 3x
distance by other plane = (x-120)*3
..
3x+3(x-120)=2400
3x+3x-360=2400
6x=2760
x=460 speed of west bound plane. 
East bound plane speed =460-120= 340 mph 
CHECK effective speed = 340+460 = 800 mph
distance = 2400 miles
time = distance / speed = 2400/800 = 3 hours