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Find the speed of the plane

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2 Answers

Let x be the speed of the plane in the still air, and w be the wind speed.
Balance by the time,
420/(x+w) = 350/(x-w)
Cross multiplication after canceling 70 gives,
6(x-w) = 5(x+w)
=> x = 11w = 11*15 = 165 mph <==Answer
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Attn: Here I assumed w = 15 mph.
 
Hi T -- since the wind makes a 70 mile difference, the wind hinders the 350mi leg by 35mi ... "still air" trip would be 385mi => the equivalent of 11 "winds" ...
"still air" mph = 11 "wind" mph ==> 165mph ...  Regards, ma'am :)