GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 15 Dec 2019, 03:47

### 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

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

# Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t

Author Message
TAGS:

### Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 59727
Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

12 Mar 2015, 06:52
1
1
00:00

Difficulty:

5% (low)

Question Stats:

90% (01:17) correct 10% (01:27) wrong based on 115 sessions

### HideShow timer Statistics

Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill the same pool. How long does it take them to fill the same pool if they are working simultaneously?

A. 7 hr
B. 10 hr
C. 12 hr
D. 14 hr
E. 18 hr

Kudos for a correct solution.

_________________
Manager
Status: Kitchener
Joined: 03 Oct 2013
Posts: 87
Concentration: Finance, Finance
GPA: 2.9
WE: Education (Education)
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

12 Mar 2015, 07:23
Pump X takes 28 hours to fill a pool, so X rates is 1/28

Pump Y takes 21 hours to fill a pool, so Y rates is 1/21

working togther=(1/21+1/28)*t=1
7/84*t=1
t=84/7 =14
Math Expert
Joined: 02 Aug 2009
Posts: 8325
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

12 Mar 2015, 09:05
1
23a2012 wrote:
Pump X takes 28 hours to fill a pool, so X rates is 1/28

Pump Y takes 21 hours to fill a pool, so Y rates is 1/21

working togther=(1/21+1/28)*t=1
7/84*t=1
t=84/7 =14

hi,
ans C..
1/21+1/28=7/84..
time taken =84/7=12...
_________________
Intern
Joined: 27 Nov 2014
Posts: 43
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

13 Mar 2015, 13:41
1
Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill the same pool. How long does it take them to fill the same pool if they are working simultaneously?

A. 7 hr
B. 10 hr
C. 12 hr
D. 14 hr
E. 18 hr

Short and quick approach for this type of questions is :

$$\frac{A X B}{A + B}$$

--> $$\frac{28 X 21}{28 + 21}$$

= 12 ans

Hence C

Regards
SG
Manager
Joined: 27 Dec 2013
Posts: 193
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

14 Mar 2015, 08:04
1
1/28+ 1/21 = 1/12

Simultaneous working= 12.

Bunuel wrote:
Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill the same pool. How long does it take them to fill the same pool if they are working simultaneously?

A. 7 hr
B. 10 hr
C. 12 hr
D. 14 hr
E. 18 hr

Kudos for a correct solution.
Math Expert
Joined: 02 Sep 2009
Posts: 59727
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

15 Mar 2015, 22:17
Bunuel wrote:
Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill the same pool. How long does it take them to fill the same pool if they are working simultaneously?

A. 7 hr
B. 10 hr
C. 12 hr
D. 14 hr
E. 18 hr

Kudos for a correct solution.

MAGOOSH OFFICIAL SOLUTION:

Express the rates of pumps in terms of (pools/hours). Pump X operates at rate of 1/28, and pump Y operates at 1/21. When the pumps work together, we add the rates.
Attachment:

wp_img10.png [ 4.43 KiB | Viewed 3616 times ]

So, combined, the pumps operate at a rate of 1/12, which means they can fill a pool in 12 hr.

_________________
SVP
Status: The Best Or Nothing
Joined: 27 Dec 2012
Posts: 1727
Location: India
Concentration: General Management, Technology
WE: Information Technology (Computer Software)
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

17 Mar 2015, 03:27
Combined rate $$= \frac{1}{21} + \frac{1}{28} = \frac{1}{12}$$

Time required = 12 Hours

ISB School Moderator
Joined: 08 Dec 2013
Posts: 620
Location: India
Concentration: Nonprofit, Sustainability
Schools: ISB '21
GMAT 1: 630 Q47 V30
WE: Operations (Non-Profit and Government)
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

25 May 2019, 05:26
Bunuel wrote:
Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill the same pool. How long does it take them to fill the same pool if they are working simultaneously?

A. 7 hr
B. 10 hr
C. 12 hr
D. 14 hr
E. 18 hr

Kudos for a correct solution.

28x = 21y = (x+y)k, we have to find k.

x/y=21/28

Now Considering equation 21y = (x+y)k, putting x/y=21/28
we get k=12, so 12 hours. C
GMAT Club Legend
Joined: 18 Aug 2017
Posts: 5484
Location: India
Concentration: Sustainability, Marketing
GPA: 4
WE: Marketing (Energy and Utilities)
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t  [#permalink]

### Show Tags

26 May 2019, 02:49
Bunuel wrote:
Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill the same pool. How long does it take them to fill the same pool if they are working simultaneously?

A. 7 hr
B. 10 hr
C. 12 hr
D. 14 hr
E. 18 hr

Kudos for a correct solution.

comined rate ; 1/21+1/28 = 58/49 ; 12 hrs
IMO C
Re: Pump X takes 28 hours to fill a pool. Pump Y takes 21 hours to fill t   [#permalink] 26 May 2019, 02:49
Display posts from previous: Sort by