Question 715363
a. At what speed did Charles Lindbergh's plane fly?
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Let the answer be x mph.
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Today, commercial airliners fly from New York to Paris five times as fast, 
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Therefore commercial airliners fly 5x mph
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or 400 miles/hour faster than Lindbergh's plane.
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Therefore commercial airliners also fly x+400 mph.

So 5x mph must equal x+400 mph, so

           5x = x + 400
           4x = 400
            x = 100

So Lindberg flew at 100 mph. 
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b. At what speed do commercial airplanes fly today? 
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5 times as fast as Lindberg, or 5×100 or 500 mph. Or you can say 
400 mph faster then 100 mph or 500 mph.  Either way you get 500 mph. 
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c. If it took Lindbergh about 36 hours to cross the Atlantic, what 
distance did he fly?
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Distance = Speed × time = 100×36 = 3600 miles
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d. How long does it take a commercial airplane today to fly across 
the Atlantic?
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TIME = DISTANCE÷SPEED = 3600÷500 = 7.2 hours = 7 hours 12 minutes

You can also get that answer by taking {{{1/5}}}th of 36 hours,
because going 5 times as fast will take only {{{1/5}}}th of the 36
hours that Lindberg took, and {{{1/5}}}th of 36 is also 7.2 hours,
or 7 hours 12 minutes.

Edwin</pre>