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The expenses of a hostel are partly fixed and partly [#permalink]

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14 Aug 2012, 11:20

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The expenses of a hostel are partly fixed and partly variable. It costs $120 per head per day if there are 20 students and $110 per head per day if there are 25 students. What will be the cost per head per day if the number of students increases to 40?

The expenses of a hostel are partly fixed and partly variable. It costs $120 per head per day if there are 20 students and $110 per head per day if there are 25 students. What will be the cost per head per day if the number of students increases to 40?

A. 105 B. 100 C. 95 D. 80 E. 75

Say the fixed cost is \($x\) and the variable cost is \($y\) per head. Then:

\(cost \ per \ head=\frac{total \ cost}{# \ of \ students}=\frac{x+20y}{20}=120\) --> \(x+20y=20*120\);

\(cost \ per \ head=\frac{total \ cost}{# \ of \ students}=\frac{x+25y}{25}=110\) --> \(x+25y=25*110\);

Subtract the first equation from the second: \(5y=25*110-20*120\) --> \(y=70\) --> \(x=1,000\).

Therefore the cost per head per day for 40 students will be \(\frac{x+40y}{y}=\frac{1,000+40*70}{40}=95\).

Re: The expenses of a hostel are partly fixed and partly [#permalink]

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18 Aug 2012, 07:02

Bunuel wrote:

NYC5648 wrote:

The expenses of a hostel are partly fixed and partly variable. It costs $120 per head per day if there are 20 students and $110 per head per day if there are 25 students. What will be the cost per head per day if the number of students increases to 40?

A. 105 B. 100 C. 95 D. 80 E. 75

Say the fixed cost is \($x\) and the variable cost is \($y\) per head. Then:

\(cost \ per \ head=\frac{total \ cost}{# \ of \ students}=\frac{x+20y}{20}=120\) --> \(x+20y=20*120\);

\(cost \ per \ head=\frac{total \ cost}{# \ of \ students}=\frac{x+25y}{25}=110\) --> \(x+25y=25*110\);

Subtract the first equation from the second: \(5y=25*110-20*120\) --> \(y=70\) --> \(x=1,000\).

Therefore the cost per head per day for 40 students will be \(\frac{x+40y}{y}=\frac{1,000+40*70}{40}=95\).

Answer: C.

Great explanation

Because he's the hero GMATclub Needs, but don't deserve. So, we'll ask him 700level questions, because he can take it. He's a silent guardian. A watchful protector. A Bunuel.

(in case you didn't notice it, I modified a quote from the movie "The dark Night"

Re: The expenses of a hostel are partly fixed and partly [#permalink]

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25 Feb 2014, 09:59

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: The expenses of a hostel are partly fixed and partly [#permalink]

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28 Feb 2014, 02:17

Bunuel wrote:

NYC5648 wrote:

The expenses of a hostel are partly fixed and partly variable. It costs $120 per head per day if there are 20 students and $110 per head per day if there are 25 students. What will be the cost per head per day if the number of students increases to 40?

A. 105 B. 100 C. 95 D. 80 E. 75

Say the fixed cost is \($x\) and the variable cost is \($y\) per head. Then:

\(cost \ per \ head=\frac{total \ cost}{# \ of \ students}=\frac{x+20y}{20}=120\) --> \(x+20y=20*120\);

\(cost \ per \ head=\frac{total \ cost}{# \ of \ students}=\frac{x+25y}{25}=110\) --> \(x+25y=25*110\);

Subtract the first equation from the second: \(5y=25*110-20*120\) --> \(y=70\) --> \(x=1,000\).

Therefore the cost per head per day for 40 students will be \(\frac{x+40y}{y}=\frac{1,000+40*70}{40}=95\).

Answer: C.

Can you kindly explain how 20y & 25y are taken?
_________________

The expenses of a hostel are partly fixed and partly variable. It costs $120 per head per day if there are 20 students and $110 per head per day if there are 25 students. What will be the cost per head per day if the number of students increases to 40?

A. 105 B. 100 C. 95 D. 80 E. 75

Say the fixed cost is \($x\) and the variable cost is \($y\) per head. Then:

\(cost \ per \ head=\frac{total \ cost}{# \ of \ students}=\frac{x+20y}{20}=120\) --> \(x+20y=20*120\);

\(cost \ per \ head=\frac{total \ cost}{# \ of \ students}=\frac{x+25y}{25}=110\) --> \(x+25y=25*110\);

Subtract the first equation from the second: \(5y=25*110-20*120\) --> \(y=70\) --> \(x=1,000\).

Therefore the cost per head per day for 40 students will be \(\frac{x+40y}{y}=\frac{1,000+40*70}{40}=95\).

Answer: C.

Can you kindly explain how 20y & 25y are taken?

The variable cost is \($y\) per head, thus for 20 students it's 20y and for 25 students it's 25y.

Re: The expenses of a hostel are partly fixed and partly [#permalink]

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28 Jun 2014, 05:49

I solved the problem using an approach similar to Bunuel's. However, it took me 2:31 min to read the question and solve the problem. I wonder if there is a faster way to solve this problem because these additional seconds (~31 seconds) all add up.

My steps: F + 20 * V = 120 * 20 F + 25 * V = 110 * 25

5 * V = 50 (55 - 12 * 4) 5 * V = 50 * 7 V = 70 F = 2400 - 1400 = 1000 Total = 1000 + 70 * 40 = 3800 At this time I looked at the answer choice and did not see anything matching and quickly realized I need per head cost So I divided the above result by 40 to get 95.

I could potentially save few seconds if I am more alert but I still feel the above steps would take me 2 min or more.

I would like to know how much time other people are taking to solve the same problem

Re: The expenses of a hostel are partly fixed and partly [#permalink]

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04 Jul 2015, 10:46

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
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