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# The Lirr River runs from Rosedale in the west to Oceanside in the east

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Joined: 02 Sep 2009
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The Lirr River runs from Rosedale in the west to Oceanside in the east  [#permalink]

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31 Jan 2019, 23:52
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Difficulty:

35% (medium)

Question Stats:

76% (01:55) correct 24% (03:17) wrong based on 39 sessions

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The Lirr River runs from Rosedale in the west to Oceanside in the east with the current moving at an average of 10 miles per hour. Sasha is traveling by motorboat from Oceanside to Rosedale and back. If the water were not moving, Sasha’s motorboat would travel at an average speed of 20 miles per hour. Given the current, what was Sasha’s actual average speed, in miles per hour, for the round trip?

A. 10
B. 15
C. 16
D. 17.5
E. 18

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Re: The Lirr River runs from Rosedale in the west to Oceanside in the east  [#permalink]

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01 Feb 2019, 00:17
Bunuel wrote:
The Lirr River runs from Rosedale in the west to Oceanside in the east with the current moving at an average of 10 miles per hour. Sasha is traveling by motorboat from Oceanside to Rosedale and back. If the water were not moving, Sasha’s motorboat would travel at an average speed of 20 miles per hour. Given the current, what was Sasha’s actual average speed, in miles per hour, for the round trip?

A. 10
B. 15
C. 16
D. 17.5
E. 18

While going from Oceanside to Rosedale, the net speed = 20-10 (current is against the speed)

While going from Rosedale to Oceanside, the net speed = 20+10 (current is with the speed)

Average Speed = Total Distance / Total Time = $$\frac{D+D}{[(D/10)+(D/30)]}$$

Average Speed = 2D*30 / 4D = 15

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The Lirr River runs from Rosedale in the west to Oceanside in the east  [#permalink]

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01 Feb 2019, 11:50
Bunuel wrote:
The Lirr River runs from Rosedale in the west to Oceanside in the east with the current moving at an average of 10 miles per hour. Sasha is traveling by motorboat from Oceanside to Rosedale and back. If the water were not moving, Sasha’s motorboat would travel at an average speed of 20 miles per hour. Given the current, what was Sasha’s actual average speed, in miles per hour, for the round trip?

A. 10
B. 15
C. 16
D. 17.5
E. 18

downstream speed= 20+10; 30
up stream speed = 20-10 ; 10
2ab/a+b
2* 30*10/40
15
IMO B
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The Lirr River runs from Rosedale in the west to Oceanside in the east   [#permalink] 01 Feb 2019, 11:50
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