Boat/Rowing Problem !!

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Taran: Senior | Next Rank: 100 Posts; Posts: 57; Joined: Fri Oct 01, 2010 7:34 am; Location: Pune, India; Thanked: 1 times; GMAT Score:680

Boat/Rowing Problem !!

by Taran » Sat Dec 11, 2010 10:56 pm

Q - A man can row at 10km/hr in still water. If it takes a total of 5 hours for him to go to a place 24 km away and return, then find the speed of the water current in km/hr.

A - 2
B - 3.5
C - 5
D - 7.2
E - 8

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Source: Beat The GMAT — Problem Solving | Sat Dec 11, 2010 10:56 pm

goyalsau: Legendary Member; Posts: 866; Joined: Mon Aug 02, 2010 6:46 pm; Location: Gwalior, India; Thanked: 31 times

by goyalsau » Sun Dec 12, 2010 12:39 am

Taran wrote:Q - A man can row at 10km/hr in still water. If it takes a total of 5 hours for him to go to a place 24 km away and return, then find the speed of the water current in km/hr.

A - 2
B - 3.5
C - 5
D - 7.2
E - 8

We can plug in answer choices,

10 + 2 = 12 km/hour

24/12 = 2 hour

10 - 2 = 8 km/hour

24/8 = 3 hour

2 + 3 = 5 hours

Option A

Saurabh Goyal
[email protected]
-------------------------

EveryBody Wants to Win But Nobody wants to prepare for Win.

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Taran: Senior | Next Rank: 100 Posts; Posts: 57; Joined: Fri Oct 01, 2010 7:34 am; Location: Pune, India; Thanked: 1 times; GMAT Score:680

by Taran » Sun Dec 12, 2010 12:50 am

Thanks goyalsau !!!

Putting in the answers was a good way to go about here, but what if the correct answer was E. I would have used lot of time to come to check option E. Did you pick to test A randomly or was there any intuitive reason?

Also, how would you solve this the normal way (w/o putting the values)?

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goyalsau: Legendary Member; Posts: 866; Joined: Mon Aug 02, 2010 6:46 pm; Location: Gwalior, India; Thanked: 31 times

by goyalsau » Sun Dec 12, 2010 1:54 am

Taran wrote:Thanks goyalsau !!!

Putting in the answers was a good way to go about here, but what if the correct answer was E. I would have used lot of time to come to check option E. Did you pick to test A randomly or was there any intuitive reason?

Also, how would you solve this the normal way (w/o putting the values)?

Let the speed of water we x

While with the water speed must be 10 + x

while coming back it must be 10 - x

total time is 5 hours.

24/(10+x ) + 24/ ( 10 - x ) = 5

10 + x + 10 - x = 100 - x^2 ( 5/24 )

480 = 500 - 5x^2

x ^2 = 4

x = 2

Speed of water is 2 km/hour

Saurabh Goyal
[email protected]
-------------------------

EveryBody Wants to Win But Nobody wants to prepare for Win.

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Taran: Senior | Next Rank: 100 Posts; Posts: 57; Joined: Fri Oct 01, 2010 7:34 am; Location: Pune, India; Thanked: 1 times; GMAT Score:680