<PRE><B>Question 647707</B>
a plane flies 400 miles with the wind and 300 miles against the wind in the
 same length of time. 
If the speed of the wind is 25 mph, what is the speed of the plane in still air?
:
Let s = speed in still air
then
(s-25) = ground speed against the wind
(s+25) = ground speed with the wind
:
Write a time equation, time = dist/speed
:
Time with the wind = time against the wind
{{{400/((s+25))}}} = {{{300/((s-25))}}}
cross multiply
400(s-25) = 300(s+25)
400s - 10000 = 300s + 7500
400s - 300s = 7500 + 10000
100s = 17500
s = 17500/100
s = 175 mph in still air
:
:
Check this by finding the actual time each way
400/200 = 2 hr
300/150 = 2 hr</PRE>