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A plane takes off from New York and travels a total distance of 1440KM

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A plane takes off from New York and travels a total distance of 1440KM  [#permalink]

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19 Aug 2017, 11:03
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78% (02:38) correct 22% (03:16) wrong based on 88 sessions

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A plane takes off from New York and travels a total distance of 1440 KMs to its destination at an average speed of 80 m/s. It then returns along the same route at twice its original speed. How much time does the trip takes? (Note 1000 m = 1 KM)

A. 0 hours 30 minutes
B. 2 hours 20 minutes
C. 5 hours
D. 7 hours 30 minutes
E. 10 hours

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Re: A plane takes off from New York and travels a total distance of 1440KM  [#permalink]

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19 Aug 2017, 11:08
While moving from New York to its destination,
Distance = 1440
Speed = 80m/s = 80*$$\frac{18}{5}$$ km/h = 16*18 km/h
Time = $$\frac{1440}{16*18}$$ = 5 hrs.
While return speed gets doubled so time will get halved. i.e. 2.5 hours
Total time = 5+2.5 = 7.5 hours. Option D
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A plane takes off from New York and travels a total distance of 1440KM  [#permalink]

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19 Aug 2017, 11:18
Formula used :Time = $$\frac{Distance}{Speed}$$

Given data:
$$Distance = 1440km$$ | $$Speed = 80 m/s$$

In order to convert a speed from m/s to km/hr, we must multiply it by a factor of $$\frac{18}{5}$$

Speed in km/hour : $$80*\frac{18}{5} = 288$$ km/hr

Therefore, the speed on the other side is $$2*288 = 576$$ km/hr

Time taken to cover the distance : $$\frac{1440}{288} + \frac{1440}{576} = 5 + \frac{5}{2}$$ = 7 hours and 30 minutes(Option D)
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Re: A plane takes off from New York and travels a total distance of 1440KM  [#permalink]

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19 Aug 2017, 19:43
I think this method saves time:

Step 1: Transform 80 meter per second into kilometers per hour.

-The plane travels 80 meters in 1 second, so it travels 4800 meters in one minute (80 meters x 60 seconds).
-4800 meters per minute are 4.8 kilometers per minute.
-The plane travels 4.8 kilometers per minute, so it travels 288 kilometers per hour (4.8 kilometers x 60 minutes).

Step 2: The time it takes the plane to travel is the distance (1440) divided by the rate (288km per hour). NEVER divide such kind of numbers, approximations are time-saving. 1440km are close to 1500, and 288km per hour is close to 300km per hour. 1500km divided by 300km per hour is 5 hours.

Step 3: The time it took the plane to travel 1440 was approx. 5 hours. But the second time the plane flew at double the rate. Therefore, the second trip took 2.5 hours. 5 + 2.5 is 7.5. The answer is D.
Re: A plane takes off from New York and travels a total distance of 1440KM   [#permalink] 19 Aug 2017, 19:43
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