Question 1193696
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Let x be the plane speed (mph).<br>
Then its speed with the wind is x+10, and its speed against the wind is x-10.<br>
The time for flying 3869 miles with the wind is 3869/(x+10); the time for flying 3869 miles against the wind is 3869/(x-10).<br>
The time for flying with the wind was 20 hours less than the time for flying against the wind:<br>
{{{3869/(x-10)-3869/(x+10)=20}}}<br>
Multiply everything by the common denominator:<br>
{{{3869(x+10)-3869(x-10)=20(x+10)(x-10)}}}
{{{3869x+38690-3869x+38690=20(x^2-100)}}}
{{{77380=20x^2-2000}}}
{{{79380=20x^2}}}
{{{x^2=3969}}}
{{{x=63}}}<br>
ANSWER: The speed of the plane in still air is x = 63mph<br>