Question 423288
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An airplane flew for 3 hours with a tail wind of 18 kilometers per hour. 
The return flight against the wind took 4 hours. Find the rate of the plane in still air.
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        The solution in the post by @mananth is irrelevant and inadequate to the problem.

        I came to bring a correct solution.



<pre>
Let x be the rate of the airplane in still air.

Then the rate of the airplane flying with    the wind is (x+18) km/h;

     the rate of the airplane flying against the wind is (x-18) km/h.


Write the distance equation, saying that the distance is the same with the wind
and against the wind

    3(x+18) = 4(x-18)  kilometers.


Simplify this equation and find x

    3x + 54 = 4x - 72,

    54 + 72 = 4x - 3x,

       126  =    x.


<U>?ANSWER</U>.  The rate of the airplane in still air is 126 km/h.
</pre>

Solved correctly.