Question 1201019
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If the speed of the plane in mph is x, then the speed on the first trip is x+15 and the speed on the second trip is x-20.<br>
Since the times for the two trips were the same, the ratio of the two distances is the same as the ratio of the two speeds:<br>
{{{645/540=(x+15)/(x-20)}}}
{{{540(x+15)=645(x-20)}}}
{{{540x+8100=645x-12900}}}
{{{21000=105x}}}
{{{x=200}}}<br>
ANSWER: 200 mph<br>