book Maths Puzzle Numerical Ability

My problem:
A plane flies 400 miles with the wind and 320 miles against the wind in the same length of time. If the speed of the wind is 20 mph, whqat is the speed of the plane in still air?

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book Other Question
The perimeter of a rectangle is 56 in. the length is 5 less thean the width. Find the demensions. Solve for x in these equations. Verify the solutions.


18-(x+3)-2x=4+(x-5)