Find all School-related info fast with the new School-Specific MBA Forum

It is currently 24 Oct 2014, 22:23

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

M10 #04

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
1 KUDOS received
Manager
Manager
avatar
Joined: 14 Oct 2008
Posts: 160
Followers: 1

Kudos [?]: 27 [1] , given: 0

M10 #04 [#permalink] New post 05 Nov 2008, 12:13
1
This post received
KUDOS
5
This post was
BOOKMARKED
The steamer going upstream would cover the distance between town A and town B in 4 hours and 30 minutes. The same steamer going downstream would cover the distance between the towns in 3 hours. How long would it take a raft moving at the speed of the current to float from town B to town A?

(A) 10 hours
(B) 12 hours
(C) 15 hours
(D) 18 hours
(E) 20 hours

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

I didn't quite understand the explanation by gmat club for this Qs. Any help is appreciated.
Thanks.
Kaplan Promo CodeKnewton GMAT Discount CodesManhattan GMAT Discount Codes
5 KUDOS received
Manager
Manager
avatar
Joined: 11 Apr 2008
Posts: 202
Followers: 2

Kudos [?]: 17 [5] , given: 1

Re: m10 -#4 [#permalink] New post 05 Nov 2008, 12:41
5
This post received
KUDOS
Let the speed of steamer=s
speed of current=c
Total distance covered=d
So the equation 1 is d/(s+c)=3
Equation 2 is d/(s-c)=9/2
(when the steamer and current in same direction then the resultant speed is added and when the steamer direction and current direction in opposite direction the resultant speed is deducted.)

Solving both equation you get s=5c
and applying the value of s in equation 1 you get the d=18c
So it will take 18 hr.
_________________

Nobody dies a virgin, life screws us all.

Intern
Intern
avatar
Joined: 15 Nov 2009
Posts: 29
Schools: Kelley
Followers: 0

Kudos [?]: 6 [0], given: 4

Re: M10 #04 [#permalink] New post 04 Dec 2009, 07:36
Say the distance between A and B is 4.5 miles.

Moving from A to B and 1 mile/hr (4.5 hours expended).
Move from B to A at 1.5 mile/hr (3 hours expended).

So from B to A, the downstream acceleration provided by water is 0.5 miles/hour. So if the engines were shut-off the boat would float down 4.5 miles at 0.5 m/h - so it would take 9 hours.

9 is not a choice in the answers provided, why is my reasoning above wrong?
11 KUDOS received
Manager
Manager
User avatar
Joined: 29 Oct 2009
Posts: 211
Followers: 68

Kudos [?]: 658 [11] , given: 18

Re: M10 #04 [#permalink] New post 04 Dec 2009, 08:34
11
This post received
KUDOS
The quickest and most accurate way to solve these problems is through the method of tabular representation.

Note for table : The values in black are those that have been given and the values in blue are those that have been calculated.
Attachment:
P1.png
P1.png [ 21.2 KiB | Viewed 6205 times ]


Since we want to find out the time taken by the raft to go from Town A to Town B, let us assign it as variable 'x'.

Now let us assume speed of boat in calm water to be 'b' and speed of current to be 's'. This gives us the expressions we require for speed in all three cases.

Note : The first thing that should strike us in this problem is that the distances are all the same. This implies that the solution will in all probability lie in equating the expression for the distances.

In row 1, we are given the time and we know that since the boat is traveling upstream, it's speed must be 'b - s'. Thus we can form an expression for the distance.

Distance(1) = Speed(1) * Time(1) = (b - s)*4.5

In row 2, we are given the time and we know that since the boat is traveling downstream, it's speed must be 'b + s'. Thus we can form an expression for the distance.

Distance(2) = Speed(2) * Time(2) = (b + s)*3

Now let us use our knowledge of the distances being equal in order to get an expression for 'b' in terms of 's'. This can be done by equating Distance(1) and Distance(2) since they are the same.

(b - s)*4.5 = (b + s)*3 ---> b = 5s

Now lets move on to row 3. We have assumed the time taken to be 'x' and we know the speed is that of the current 's'. Thus we can obtain an expression in for the distance in terms of 's' and 'x'.

Distance(3) = Speed(3) * Time(3) = s*x

Again, we know that this must be equal to both Distance(1) and Distance(2). So let us equate it with any one of them to obtain an expression for 'x' in terms of 'b' and 's'.

Equating it to Distance(2) we get :

(b + s)*3 = s*x ---> 3b + 3s = s*x ---> Substituting b = 5s ---> 18s = s*x ---> x = 18 hours.

OR

In case we would have equated it to Distance(1) we would still have got the same result :

(b - s)*4.5 = s*x ---> 4.5b - 4.5s = s*x ---> Substituting b = 5s ---> 22.5s - 4.5s = s*x ---> x = 18 hours.


Answer : 18 hours.





junker wrote:
Say the distance between A and B is 4.5 miles.

Moving from A to B and 1 mile/hr (4.5 hours expended).
Move from B to A at 1.5 mile/hr (3 hours expended).

So from B to A, the downstream acceleration provided by water is 0.5 miles/hour. So if the engines were shut-off the boat would float down 4.5 miles at 0.5 m/h - so it would take 9 hours.

9 is not a choice in the answers provided, why is my reasoning above wrong?



Moving from A to B and 1 mile/hr (4.5 hours expended)
Move from B to A at 1.5 mile/hr (3 hours expended)

Your reasoning is correct till this point. However, you cannot subtract one speed from the other to get the speed of the current. In one case it is the speed of the boat - speed of current while in the other case it is the speed of boat + speed of current.

Thus you will have the following two expressions :

b + s = 1.5

b - s = 1


Solving them you will get b = 1.25 mph and s = 0.25 mph

Then you can calculate the time taken by raft to be = 4.5/0.25 = 18 hours.


Although this method is not wrong, there is a lot of scope for silly mistakes (as you would have realized) if your concepts are not one hundred percent clear.

Personally, I believe that the most fool proof way to solve these problems is through tabular representation. (At least until your concepts become strong and maybe even then).

You can check out my post on these types of word problems (you'll find the link below). You might find it helpful.

Cheers.
_________________

Click below to check out some great tips and tricks to help you deal with problems on Remainders!
compilation-of-tips-and-tricks-to-deal-with-remainders-86714.html#p651942

Word Problems Made Easy!
1) Translating the English to Math : word-problems-made-easy-87346.html
2) 'Work' Problems Made Easy : work-word-problems-made-easy-87357.html
3) 'Distance/Speed/Time' Word Problems Made Easy : distance-speed-time-word-problems-made-easy-87481.html

Manager
Manager
User avatar
Joined: 13 Dec 2009
Posts: 79
Followers: 1

Kudos [?]: 32 [0], given: 20

Re: M10 #04 [#permalink] New post 12 Feb 2010, 22:03
Great explanation sriharimurthy! Kudos!
Forum Moderator
Forum Moderator
User avatar
Status: doing good things...
Joined: 02 Jul 2009
Posts: 1245
Concentration: Entrepreneurship, Finance
GMAT 1: Q V
GMAT 2: 690 Q49 V35
GPA: 3.77
WE: Corporate Finance (Commercial Banking)
Followers: 157

Kudos [?]: 527 [0], given: 531

GMAT ToolKit User Premium Member Reviews Badge
Re: M10 #04 [#permalink] New post 29 Sep 2010, 04:03
I am wondering why does the substitution not work for this problem?
_________________

Follow me, if you find my explanations useful.

Audaces fortuna juvat!

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Manager
Manager
avatar
Joined: 23 Oct 2010
Posts: 87
Location: India
Followers: 3

Kudos [?]: 21 [0], given: 6

Re: M10 #04 [#permalink] New post 06 Dec 2010, 08:24
D.

let the speed of steamer be s and current be c
Respective speeds =>
upstream = s - c
downstream = s + c

distances covered being equal ->
(s - c) *270 = (s+c)*180
(s-c)/(s+c) = 2/3 => s = 5c

s + c = 5c + c = 6c
3 * 6c = x * c
=> x = 18 hrs
Manager
Manager
avatar
Joined: 21 Nov 2010
Posts: 133
Followers: 0

Kudos [?]: 5 [0], given: 12

Re: M10 #04 [#permalink] New post 11 Dec 2011, 16:13
This question seems pretty confusing. Anyone know where this one would rank?
Manager
Manager
avatar
Joined: 20 Jan 2011
Posts: 65
Followers: 1

Kudos [?]: 1 [0], given: 8

Re: M10 #04 [#permalink] New post 12 Dec 2011, 06:59
Good explanation Sriharimurthy. One needs to understand the concept of upstream and downstream to convert it into equations.
Intern
Intern
avatar
Status: single
Joined: 13 May 2011
Posts: 19
Location: India
Concentration: Entrepreneurship, Strategy
GPA: 3
WE: Manufacturing and Production (Other)
Followers: 0

Kudos [?]: 16 [0], given: 6

Re: M10 #04 [#permalink] New post 10 Dec 2012, 08:02
Let the speed of the boat be B and that of the stream be S. An ideal raft will travel at the speed of the current so time = distance/S
Upstream - the effective velocity of the boat = velocity of boat in still water - velocity of stream = B-S
time = D/(B-S) or D = 4.5(B-S) ......I
Downstream - the effective velocity of the boat = velocity of boat in still water + velocity of stream = B+S
time = D/(B+S) or D = 3(B+S) .....II

equating both values of D we get B = 5S
substitute the value of B in any of the equation to get D= 18S

Therefore, time taken by ideal raft = D/S = 18S/S = 18 hours.
Current Student
avatar
Joined: 24 Apr 2012
Posts: 38
GMAT 1: 660 Q48 V33
WE: Information Technology (Health Care)
Followers: 0

Kudos [?]: 4 [0], given: 35

Re: M10 #04 [#permalink] New post 20 Dec 2012, 14:11
I did this as follows -

s=> speed f boat and r=> speed of river

s-r = 4.5
s+r = 3

solving we get s=7.5/2
r=1.5/2

D= 4.5 * (s-r) = 13.5

Now my issue is understanding the problem it says -

How long would it take a raft moving at the speed of the current to float from town B to town A

How would you know the time is d/r and not d/2*r
I thought since the speed of the raft = current the time taken will be half..again if this is the case the answer would be straight forward (i know) .. but at first glance that is what I understood.. can anyone please me understand the wordings
Re: M10 #04   [#permalink] 20 Dec 2012, 14:11
    Similar topics Author Replies Last post
Similar
Topics:
Experts publish their posts in the topic M10-04 Bunuel 1 15 Sep 2014, 23:41
17 M10 #04 gameCode 10 05 Nov 2008, 12:13
5 Experts publish their posts in the topic m10 q18 georgechanhc 16 29 Dec 2008, 22:01
25 Experts publish their posts in the topic M10 Q35 gk2k2 29 21 Dec 2008, 10:13
25 Experts publish their posts in the topic m10 Q13 smarinov 18 20 Dec 2008, 15:42
Display posts from previous: Sort by

M10 #04

  Question banks Downloads My Bookmarks Reviews Important topics  

Moderators: WoundedTiger, Bunuel



GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.