Question 1140355
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The speed against the wind is 400/2.5 = 160km/hr; the speed with the wind is 200/(5/6) = 240km/hr.<br>
The speed with the wind is the sum of the plane's speed and the wind speed; the speed against the wind is the difference between the plane's speed and the wind speed.<br>
So this is one of a large number of different kinds of problem where you need to find two numbers if you know the sum of the two numbers and the difference between them.<br>
Of course algebra is a good way to solve a problem like that.  However, since this kind of problem is encountered frequently, you can save time and effort by learning to solve the problem quickly with logical reasoning.<br>
You know the sum of the plane's speed and the wind speed is 240km/hr, and you know that the difference between the two speeds is 160km/hr.  Logical reasoning (picture the numbers on a number line) tells you that the plane's speed is halfway between those two speeds; and then the wind speed is the difference between the plane's speed and each of those two speeds.  So...<br>
The plane's speed is halfway between 160km/hr and 240km/hr: 200 km/hr.  And the wind speed is the difference between 200 and 240km/hr (or the difference between 200 and 160km/hr), which is 40km/hr.<br>
ANSWER (by logical reasoning instead of formal algebra): Plane speed 200km/hr; wind speed 40km/hr.