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Hi Devmitra,
I didnot understand you solution . could you elaborate how you are forming the equation for g'(x)
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The approach i used may take time but i think that it is the only approach i could use. ( Without hit and trial )

Total fare = g(x) = ( 54 - (x/32))^2 * x = (x^3 - 3456x^2 + (1728)^2*x)/1024

The numbers may seem large but solving the equation gets easier once you differentiate.

x^2 - 2304x + 1728^2 = 0

Roots = (-b +- sqrt(b^2 - 4ac ))/2a

You get 1728 and 576 as the roots for the maxima and minima of the function.

Since 1728>900, you get the answer as 576.

I solved this in 3 minutes wich is more than the average time per question.

But inserting the options and finding may take equivalent time if you are not good with numbers.
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CONCEPT: Functions and derivatives
At the point maximum, \(f'(x)=0\)

SOLUTION: Given, the number of people per trip is x.

The fare for each person =\([(54−(\frac{x}{32})]^2\)

The revenue from x people =\(x[(54−(\frac{x}{32})]^2 =g(x)\)

where \(g(x)=[x(54−(\frac{x}{32})]^2\)

When the revenue is the maximum, the marginal revenue equal to zero.
=>\(g′(x)=[54−(\frac{x}{32})]^2−(\frac{2x}{32})[(54-(\frac{x}{32})]\)

For the maximum value of \(g(x)\):
\(g′(x)=0\)
⇒\( 54−(\frac{x}{32})=0 and (54−\frac{3x}{32})=0\)
⇒ x=1728,x=576

Given the capacity of the train is 900, the number of people per trip = 576 (C)
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I could be wrong, but Calculus may be out of scope for GMAT. there has to be a simpler way to solve this?

chetan2u Bunuel
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wishmasterdj
I could be wrong, but Calculus may be out of scope for GMAT. there has to be a simpler way to solve this?

chetan2u Bunuel


This is a CAT question for the entrance to IIMs in India.
Calculus is not something that is tested directly. However if some of these types are seen, they will surely have some other way to be solved.
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Bunuel
A Metro train from Mumbai to Gurgaon has capacity to board 900 people. The fare charged (in Rs) is defined by the function: \(f=(54−\frac{x}{32})^2\) where x is the number of the people per trip. How many people per trip will make the revenue for the trip maximum?

A. 1728
B. 592
C. 576
D. 484
E. 364


Are You Up For the Challenge: 700 Level Questions
x*(54-x/32 )^2 =0
maximising the function

=>(54−x/32)^2 -x/32*(54-x/32) =0

=>x=32*18
=> x=24 *24
=>x=576

Therefore IMO C
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