<PRE><B>Question 264348</B>
450 miles with the wind = 450/(x+40)
300 against the wind = 300/(x-40)
450/(x+40) = 300/(x-40)
multiply each side by (x+40)(x-40) 
450(x-40) = 300(x+40)
450x - 18,000 = 300x + 12,000
450x - 18,000 - 300x = 300x + 12,000 - 300x
150x - 18,000 + 18,000 = 12,000 + 18,000
150x = 30,000
divide each side by 150
x = 200
The speed of the airplane in still air is 200 mph
check:
450/(200+40) = 300/(200-40)
450/(240) = 300/(160)
15/8 = 15/8</PRE>