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If the farmer sells 75 of his chickens, his stock of feed

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If the farmer sells 75 of his chickens, his stock of feed [#permalink] New post 25 Oct 2009, 01:41
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If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned, but if he buys 100 more chickens, he will run out of feed 15 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?

A. 60
B. 120
C. 240
D. 275
E. 300
[Reveal] Spoiler: OA

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Re: Feeding the Chickens [#permalink] New post 25 Oct 2009, 11:35
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slingfox wrote:
If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned, but if he buys 100 more chickens, he will run out of feed 15 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?

A. 60
B. 120
C. 240
D. 275
E. 300


# of chickens - x
# of days - d

If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned --> Amount of feed equals xd=(x-75)(d+20);
If he buys 100 more chickens, he will run out of feed 15 days earlier than planned --> Amount of feed equals xd=(x+100)(d-15).

(x-75)(d+20)=(x+100)(d-15) --> \frac{d+20}{d-15}=\frac{x+100}{x-75} --> x=5d

xd=(x-75)(d+20) --> 5d^2=(5d-75)(d+20) --> d^2=(d-15)(d+20) --> d=60 --> x=5d=300.

Answer: E (300)

Hope it helps.
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Re: Feeding the Chickens [#permalink] New post 06 Dec 2010, 05:34
Bunuel, I have tried a different approach. Can you please explain why it didn't work? Thank you.

x ... number of chickens
k ... 1 chicken consumption per day
T ... total number of days for x chickens to live given their current number

We get the following equations:

(y - 75)/k = T +20
(y + 100)/k = T - 15
y/k = T

However, I couldn't solve the equations. I got k = 5. But substituting 5 into the equation, I couldn't solve it.

Thank you.
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Re: Feeding the Chickens [#permalink] New post 06 Dec 2010, 08:03
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slingfox wrote:
If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned, but if he buys 100 more chickens, he will run out of feed 15 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?

A. 60
B. 120
C. 240
D. 275
E. 300


or you can make your equations in this way: Let us say he has planned for d days for c chickens. According to the question,

the food 75 chicken consumed in d days will last 20 days if consumed by (c - 75) chickens
So 75d = (c - 75)20 ..... (I)

What c chickens consumed in 15 days, 100 chickens will consume in (d - 15) days
15c = 100(d - 15) ......(II)

Solve I and II to get c = 300
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Re: If the farmer sells 75 of his chickens, his stock of feed [#permalink] New post 27 Sep 2013, 08:10
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Re: If the farmer sells 75 of his chickens, his stock of feed [#permalink] New post 19 Oct 2013, 04:42
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Let Number of Chickens be X

The number of days chicken will eat a fixed stock will be inversely proportional to number of chickens. Hence
if number of days X chicken will eat the stock will be D = K/ X

If number of chicken is reduced by 75 then D+20 = K / (X-75)
If number of chicken is increaded by 100 then D-15 = K / (X+100)

Replacing D=K/X in above two equations:-

K/X + 20 = K / (X-75)
K/X - 15 = k / (X+100)

solving above two equations gives X =300.

Answer is E
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Re: Feeding the Chickens [#permalink] New post 02 Nov 2013, 09:41
Bunuel wrote:
slingfox wrote:
If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned, but if he buys 100 more chickens, he will run out of feed 15 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?

A. 60
B. 120
C. 240
D. 275
E. 300


# of chickens - x
# of days - d

If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned --> Amount of feed equals xd=(x-75)(d+20);
If he buys 100 more chickens, he will run out of feed 15 days earlier than planned --> Amount of feed equals xd=(x+100)(d-15).

(x-75)(d+20)=(x+100)(d-15) --> \frac{d+20}{d-15}=\frac{x+100}{x-75} --> x=5d

xd=(x-75)(d+20) --> 5d^2=(5d-75)(d+20) --> d^2=(d-15)(d+20) --> d=60 --> x=5d=300.

Answer: E (300)

Hope it helps.



Thank you for the explanation Bunuel.
However, I do not understand something here:
How does multiplying the number of days (d) with the number of chickens (x) give the amount of feed the farmer has ?

Shouldn't it rather be the amount of feed the chickens consume in one d days ?
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Re: Feeding the Chickens [#permalink] New post 03 Nov 2013, 10:39
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nishantsharma87 wrote:
Bunuel wrote:
slingfox wrote:
If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned, but if he buys 100 more chickens, he will run out of feed 15 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?

A. 60
B. 120
C. 240
D. 275
E. 300


# of chickens - x
# of days - d

If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned --> Amount of feed equals xd=(x-75)(d+20);
If he buys 100 more chickens, he will run out of feed 15 days earlier than planned --> Amount of feed equals xd=(x+100)(d-15).

(x-75)(d+20)=(x+100)(d-15) --> \frac{d+20}{d-15}=\frac{x+100}{x-75} --> x=5d

xd=(x-75)(d+20) --> 5d^2=(5d-75)(d+20) --> d^2=(d-15)(d+20) --> d=60 --> x=5d=300.

Answer: E (300)

Hope it helps.



Thank you for the explanation Bunuel.
However, I do not understand something here:
How does multiplying the number of days (d) with the number of chickens (x) give the amount of feed the farmer has ?

Shouldn't it rather be the amount of feed the chickens consume in one d days ?


That's because the amount of feed each chicken eats a day, say z, can be reduced on both sides:
zxd=z(x-75)(d+20) --> xd=(x-75)(d+20);
zxd=z(x+100)(d-15) --> xd=(x+100)(d-15).

Hope it's clear.
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Re: Feeding the Chickens [#permalink] New post 04 Nov 2013, 10:07
Thank you for the explanation as always Bunuel!

I find such word problems, where we have to assume some constants/variable to solve the question (that cancel out before the final solution arrives) and ALSO infer its relationship with the variables/constants given in the word problem, quite tricky (specially WORK/RATE problems! )

It'll be very helpful if you can provide some similar questions or content (or their link) to practice.

Cheers!
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Re: Feeding the Chickens [#permalink] New post 29 Dec 2013, 18:58
Bunuel wrote:
slingfox wrote:
If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned, but if he buys 100 more chickens, he will run out of feed 15 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?

A. 60
B. 120
C. 240
D. 275
E. 300


# of chickens - x
# of days - d

If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned --> Amount of feed equals xd=(x-75)(d+20);
If he buys 100 more chickens, he will run out of feed 15 days earlier than planned --> Amount of feed equals xd=(x+100)(d-15).

(x-75)(d+20)=(x+100)(d-15) --> \frac{d+20}{d-15}=\frac{x+100}{x-75} --> x=5d

xd=(x-75)(d+20) --> 5d^2=(5d-75)(d+20) --> d^2=(d-15)(d+20) --> d=60 --> x=5d=300.

Answer: E (300)

Hope it helps.


Wow! When I first saw this question, I had no clue even how to begin. :shock: Bunuel, where in the problem suggests that one should take this approach?
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Re: If the farmer sells 75 of his chickens, his stock of feed [#permalink] New post 30 Dec 2013, 03:11
slingfox wrote:
If the farmer sells 75 of his chickens, his stock of feed will last for 20 more days than planned, but if he buys 100 more chickens, he will run out of feed 15 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?

A. 60
B. 120
C. 240
D. 275
E. 300


This question is similar to work-rate questions. The key is always calculate how much work each person/machine does in 1 unit of time.

For this question, we have:
number of chickens = X
stock feed = T (days)
It means X chickens can be fed in T days --> 1 chicken eats in 1 day = 1/(XT)

1st scenario, we sell 75 chickens, we have
number of chickens = X - 75
stock feed = T + 20 (days)
--> 1 chicken eats 1 day = 1/[(X-75)(T+20)]
Because the amount of food each chicken eats in 1 day is the same:
--> 1/XT = 1/[(X-75)(T+20)]
--> XT = XT+ 20X - 75T - 1500
--> 20X - 75T - 1500 = 0

2nd scenario: we buy 100 chickens
number of chickens = X + 100
stock feed = T - 15 (days)
--> 1 chicken eats 1 day = 1/[(X+100)(T-15)]
Because the amount of food each chicken eats in 1 day is the same:
--> 1/XT = 1/[(X+100)(T-15)]
--> XT = XT -15X +100T - 1500
--> 15X +100T - 1500 = 0

Solve 2 equations
20X - 75T - 1500 = 0
15X +100T - 1500 = 0


Clearly, X = 300

Hence, E is correct.

We have
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Re: If the farmer sells 75 of his chickens, his stock of feed [#permalink] New post 18 Feb 2014, 15:50
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Its quite a journey but here we go. (q-75)(t+20) and (q+100)(t-15). Now we need to equal each to qt. So we will have 20q - 75t - 75(20) on the first equation and -15q + 100t - 100(15) on the second equation. We could simplify some terms but the point is we need to find 'q' so anyways after simplifying we can multiply the first equation by 3 and we're left with 12q - 60t -75 *(4) *(4) and multiply the second by 3 to get -9q + 60t - 100 (3)*3. So then we finally get to 3q = 900, q = 300

E is the answer

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Re: If the farmer sells 75 of his chickens, his stock of feed   [#permalink] 18 Feb 2014, 15:50
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