It is currently 23 Oct 2017, 23:06

### 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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# M13-09

Author Message
TAGS:

### Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 41909

Kudos [?]: 129564 [0], given: 12202

### Show Tags

16 Sep 2014, 00:48
Expert's post
3
This post was
BOOKMARKED
00:00

Difficulty:

25% (medium)

Question Stats:

80% (00:53) correct 20% (02:59) wrong based on 88 sessions

### HideShow timer Statistics

A certain colony of bacteria doubles every morning while every evening 1000 bacteria die. If by the end of the third day the number of bacteria was 0, what was the original size of the colony by the end of the first day?

A. 825
B. 850
C. 875
D. 910
E. 950
[Reveal] Spoiler: OA

_________________

Kudos [?]: 129564 [0], given: 12202

Math Expert
Joined: 02 Sep 2009
Posts: 41909

Kudos [?]: 129564 [0], given: 12202

### Show Tags

16 Sep 2014, 00:48
Official Solution:

A certain colony of bacteria doubles every morning while every evening 1000 bacteria die. If by the end of the third day the number of bacteria was 0, what was the original size of the colony by the end of the first day?

A. 825
B. 850
C. 875
D. 910
E. 950

Let $$x$$ denote the original size of the colony. Compose the following equation:

$$2*(2*(2*x - 1000) - 1000) - 1000 = 0$$, which simplifies to $$8x = 7000$$.

From here, $$x = 875$$.

_________________

Kudos [?]: 129564 [0], given: 12202

Intern
Joined: 21 Jan 2015
Posts: 21

Kudos [?]: 5 [0], given: 11

### Show Tags

15 Aug 2015, 04:15
1
This post was
BOOKMARKED
I think this is a high-quality question and the explanation isn't clear enough, please elaborate. I don't understand this question.
Per my understanding,
Start : 825
Day 1 morning : 1650
Day 1 evening : 650
Day 2 morning : 1300
Day 2 evening : 300
Day 3 morning 600
Day 3 evening : everything disapeared.

What is wrong ?

Kudos [?]: 5 [0], given: 11

Math Expert
Joined: 02 Sep 2009
Posts: 41909

Kudos [?]: 129564 [0], given: 12202

### Show Tags

20 Aug 2015, 09:38
Barnal wrote:
I think this is a high-quality question and the explanation isn't clear enough, please elaborate. I don't understand this question.
Per my understanding,
Start : 825
Day 1 morning : 1650
Day 1 evening : 650
Day 2 morning : 1300
Day 2 evening : 300
Day 3 morning 600
Day 3 evening : everything disapeared.

What is wrong ?

_____
Edited the question.
_________________

Kudos [?]: 129564 [0], given: 12202

Intern
Joined: 19 Aug 2015
Posts: 9

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

### Show Tags

14 Feb 2016, 16:15
based on the wording, why wouldn't it be A/B?

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

Manager
Joined: 12 Nov 2015
Posts: 61

Kudos [?]: 24 [0], given: 126

Location: Uruguay
Concentration: General Management
Schools: Goizueta '19 (A)
GMAT 1: 610 Q41 V32
GMAT 2: 620 Q45 V31
GMAT 3: 640 Q46 V32
GPA: 3.97

### Show Tags

28 Jun 2016, 17:22
intogamer wrote:
based on the wording, why wouldn't it be A/B?

Hope it helps:

Day one-
morning: 875*2 =1750
Evening: 1750- 1000= 750

Day two-
morning: 750*2= 1500
Evening: 1500-1000= 500

Day three-
morning: 500*2=1000
Evening: 1000-1000= 0 (all bacteria succumb)

Kudos [?]: 24 [0], given: 126

Manager
Joined: 03 Apr 2016
Posts: 103

Kudos [?]: 16 [0], given: 31

Location: India
Concentration: Operations, General Management
GMAT 1: 720 Q50 V37
WE: Analyst (Computer Software)

### Show Tags

20 Jul 2016, 08:01
well I tried with working backwards from Option approach, and started with option B and it worked, don't know why only option C is correct, maybe the question needs to be rephrased somehow.

Kudos [?]: 16 [0], given: 31

Manager
Joined: 12 Nov 2015
Posts: 61

Kudos [?]: 24 [0], given: 126

Location: Uruguay
Concentration: General Management
Schools: Goizueta '19 (A)
GMAT 1: 610 Q41 V32
GMAT 2: 620 Q45 V31
GMAT 3: 640 Q46 V32
GPA: 3.97

### Show Tags

20 Jul 2016, 08:51
shashanksagar wrote:
well I tried with working backwards from Option approach, and started with option B and it worked, don't know why only option C is correct, maybe the question needs to be rephrased somehow.

What should happen is that in the night of the third day, when 1000 die, you don't get exactly zero bacteria but a negative number

Kudos [?]: 24 [0], given: 126

Manager
Joined: 03 Apr 2016
Posts: 103

Kudos [?]: 16 [0], given: 31

Location: India
Concentration: Operations, General Management
GMAT 1: 720 Q50 V37
WE: Analyst (Computer Software)

### Show Tags

20 Jul 2016, 09:07
Number of Bacteria cant be negative.. when I'll wake up in the morning i'll have 0 bacteria on my plate , so its the same with 825 and 850.

Sent from my SM-N910H using Tapatalk

Kudos [?]: 16 [0], given: 31

Manager
Joined: 12 Nov 2015
Posts: 61

Kudos [?]: 24 [0], given: 126

Location: Uruguay
Concentration: General Management
Schools: Goizueta '19 (A)
GMAT 1: 610 Q41 V32
GMAT 2: 620 Q45 V31
GMAT 3: 640 Q46 V32
GPA: 3.97

### Show Tags

20 Jul 2016, 11:20
shashanksagar wrote:
Number of Bacteria cant be negative.. when I'll wake up in the morning i'll have 0 bacteria on my plate , so its the same with 825 and 850.

Sent from my SM-N910H using Tapatalk

Unfortunatelly for the bacteria no, it can't be negative,
but the question explicitly says the number of bacteria must be zero that's why 875 is the best choice, in my humble opinion, since algebraically 875 yields exactly zero.
I'm not sure this question could appear on an official GMAT since it does leave room to interpretation since bacteria cannot be inferior to zero, although it works for algebra, it wouldn't in the real world.

Kudos [?]: 24 [0], given: 126

Intern
Joined: 26 Aug 2016
Posts: 2

Kudos [?]: [0], given: 37

### Show Tags

29 Sep 2016, 07:39
Good question. Thanks for posting !

Sent from my SM-J700F using GMAT Club Forum mobile app

Kudos [?]: [0], given: 37

Intern
Joined: 07 Oct 2016
Posts: 13

Kudos [?]: 34 [0], given: 3

### Show Tags

03 Dec 2016, 05:38
why are we starting the first day morning with 2X. ?
if we assuming the day 1 of bacteria then why are we starting the doubling in the morning?

How I solved it was -
2∗(2∗(x−1000)−1000)−1000=0

i am conceptually missing out on something

Kudos [?]: 34 [0], given: 3

Manager
Joined: 12 Jun 2016
Posts: 220

Kudos [?]: 35 [0], given: 130

Location: India
WE: Sales (Telecommunications)

### Show Tags

12 Sep 2017, 21:30
Hello gmatdemolisher1234,

I think this is how this question Pans out. Hope this clear things up a bit

• Original= X
• (Original +1) Morning = 2X
• (Original+1)Evening = 2X-1000
• (Original +2)Morning = 2*(2X-1000) = 4X-2000
• (Original +2)Evening = 4X-2000 - 1000 = 4X - 3000
• (Original +3) Morning = 2*(4X - 3000) = 8X - 6000
• (Original +3) Evening = 8X - 6000 - 1000 = 8X - 7000

The stem says - 8X - 7000 = 0. From this we get X = 875. The Answer is B

Aside Bunuel:

Though I understood this question, I have a small question. Maybe I am over thinking, can you please help clarify?

If we consider Original = X, why dont we consider that 1000 Bacteria die on the original day's evening. Meaning, at the end of original day why is the bacteria count still X and not (X-1000). I am not able to understand from the stem what stops us from thinking so.

Hope I am able to communicate what I intend to say

gmatdemolisher1234 wrote:
why are we starting the first day morning with 2X. ?
if we assuming the day 1 of bacteria then why are we starting the doubling in the morning?

How I solved it was -
2∗(2∗(x−1000)−1000)−1000=0

i am conceptually missing out on something

_________________

My Best is yet to come!

Kudos [?]: 35 [0], given: 130

Math Expert
Joined: 02 Sep 2009
Posts: 41909

Kudos [?]: 129564 [0], given: 12202

### Show Tags

12 Sep 2017, 23:11
susheelh wrote:
Hello gmatdemolisher1234,

I think this is how this question Pans out. Hope this clear things up a bit

• Original= X
• (Original +1) Morning = 2X
• (Original+1)Evening = 2X-1000
• (Original +2)Morning = 2*(2X-1000) = 4X-2000
• (Original +2)Evening = 4X-2000 - 1000 = 4X - 3000
• (Original +3) Morning = 2*(4X - 3000) = 8X - 6000
• (Original +3) Evening = 8X - 6000 - 1000 = 8X - 7000

The stem says - 8X - 7000 = 0. From this we get X = 875. The Answer is B

Aside Bunuel:

Though I understood this question, I have a small question. Maybe I am over thinking, can you please help clarify?

If we consider Original = X, why dont we consider that 1000 Bacteria die on the original day's evening. Meaning, at the end of original day why is the bacteria count still X and not (X-1000). I am not able to understand from the stem what stops us from thinking so.

Hope I am able to communicate what I intend to say

gmatdemolisher1234 wrote:
why are we starting the first day morning with 2X. ?
if we assuming the day 1 of bacteria then why are we starting the doubling in the morning?

How I solved it was -
2∗(2∗(x−1000)−1000)−1000=0

i am conceptually missing out on something

The question asks about the original size of the colony by the end of the first day.
_________________

Kudos [?]: 129564 [0], given: 12202

Intern
Joined: 19 Oct 2015
Posts: 29

Kudos [?]: 2 [0], given: 8

### Show Tags

12 Sep 2017, 23:24
First day morning 875*2=1750
First day eve 1750-1000=750

Second day morning 750*2= 1500
Second day eve 1500-1000=500

Third day morning 500*2=1000
Third day eve 1000-1000=0

Sent from my A0001 using GMAT Club Forum mobile app

Kudos [?]: 2 [0], given: 8

Manager
Joined: 12 Jun 2016
Posts: 220

Kudos [?]: 35 [0], given: 130

Location: India
WE: Sales (Telecommunications)

### Show Tags

13 Sep 2017, 00:44
Thanks a Ton Bunuel for replying!!

Please bear with me for few more mins. Its just that I want to be sure I understand questions such as this correctly when I see them next.

I am still unable to see where the question asks about - "the original size of the colony by the end of the first day".

I am breaking down the question into bits to explain what I understood from each of the part. Can you please see where I am going wrong?

Quote:
A certain colony of bacteria doubles every morning while every evening 1000 bacteria die.

This is a fact to be used while calculating.

Quote:
If by the end of the third day the number of bacteria was 0

This tells us what is the Needed. Meaning what is needed at the end of third day.

Quote:
what was the original size of the colony?

The Stem says what's the original size of the colony. But does not seem to mention if this original is the Morning or the Evening. Of course if it had said - "what was the original size of the colony at the end of first day" - I would not be having this question.

I do understand I might be simply Over thinking here

_________________

My Best is yet to come!

Kudos [?]: 35 [0], given: 130

Math Expert
Joined: 02 Sep 2009
Posts: 41909

Kudos [?]: 129564 [1], given: 12202

### Show Tags

13 Sep 2017, 00:46
1
KUDOS
Expert's post
susheelh wrote:
Thanks a Ton Bunuel for replying!!

Please bear with me for few more mins. Its just that I want to be sure I understand questions such as this correctly when I see them next.

I am still unable to see where the question asks about - "the original size of the colony by the end of the first day".

I am breaking down the question into bits to explain what I understood from each of the part. Can you please see where I am going wrong?

Quote:
A certain colony of bacteria doubles every morning while every evening 1000 bacteria die.

This is a fact to be used while calculating.

Quote:
If by the end of the third day the number of bacteria was 0

This tells us what is the Needed. Meaning what is needed at the end of third day.

Quote:
what was the original size of the colony?

The Stem says what's the original size of the colony. But does not seem to mention if this original is the Morning or the Evening. Of course if it had said - "what was the original size of the colony at the end of first day" - I would not be having this question.

I do understand I might be simply Over thinking here

Edited the question to end this confusion.
_________________

Kudos [?]: 129564 [1], given: 12202

Intern
Joined: 20 Aug 2017
Posts: 30

Kudos [?]: 1 [0], given: 4

### Show Tags

15 Sep 2017, 05:54
I still find the question unclear.

"original size of the colony by the end of the first day" is ambiguous. Why not just ask for the original size of the colony at the start of day one?

By the end of the first day, the original size of 875 would have become 750, which was not a choice, which got me very confused.

I think this a poorly worded item and needs editing.

Kudos [?]: 1 [0], given: 4

Intern
Joined: 15 Aug 2017
Posts: 14

Kudos [?]: 0 [0], given: 2

### Show Tags

01 Oct 2017, 00:15
alainca wrote:
I still find the question unclear.

"original size of the colony by the end of the first day" is ambiguous. Why not just ask for the original size of the colony at the start of day one?

By the end of the first day, the original size of 875 would have become 750, which was not a choice, which got me very confused.

I think this a poorly worded item and needs editing.

I Agree. If the question intends for us to solve for the original size of the colony why not write "Original size of the colony".
End of first day made me sure we were implying the count after we had already cycled once.
Let x = original be size at the instant the cycle started (2x-1000) Brings us to the evening of the first day, which we will use for the morning of the second day. and so on for the third.

$$[2*(2*(2*x-1000)-1000)-1000]=0$$ which brings us to the correct finishing point of $$\frac{7000}{8}=x=875$$

However when we consider x = original size at END of day one, we are assuming one cycle has already passed. Thus we only cycle 2 more times.
$$2x(2x-1000)-1000=0$$
Which gives us $$4x=3000$$ which is unsolvable through any of the answer choices. (Max would have to be 750)

Kudos [?]: 0 [0], given: 2

Intern
Joined: 23 May 2017
Posts: 1

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

### Show Tags

09 Oct 2017, 14:02
alainca wrote:
I still find the question unclear.

"original size of the colony by the end of the first day" is ambiguous. Why not just ask for the original size of the colony at the start of day one?

By the end of the first day, the original size of 875 would have become 750, which was not a choice, which got me very confused.

I think this a poorly worded item and needs editing.

I agree. End of the first day implies the answer should be 750.

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

Re: M13-09   [#permalink] 09 Oct 2017, 14:02

Go to page    1   2    Next  [ 21 posts ]

Display posts from previous: Sort by

# M13-09

Moderators: Bunuel, Vyshak

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne 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®.