When the question says "r in terms of p", it implies that r should be on the left hand side of the equation and everything on the right should only in terms of p. There should be no other variable on the right hand side. Then either you can solve algebraically or plug in values.

The question says you need to find r. What is r? It is "r percent of the store’s revenues from newspaper sales was from Newspaper A"
It means: Revenue from Newspaper A = (r/100)* Total revenue
So r = (Revenue from Newspaper A/Total revenue) * 100
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If you're not sure how to proceed with this question, or if you're behind on time and you want to catch up, you can give yourself a 50-50 chance in about 20 seconds.

To do so, we'll see what happens when we use an EXTREME value for p.
Say p = 100
In other words, 100% of the newspapers sold were Newspaper A.
This means that 100% of the revenue is from Newspaper A.
In other words, when p = 100, then r = 100

At this point, we'll plug in 100 for p and see which one yields a value of 100.
Only answer choices B and D work.
B) 150(100)/(250-100) = 100 PERFECT
D) 400(100)/(500-100) = 100 PERFECT

Now take a guess (B or D) and move on.

Related Resources
The posters have demonstrated two methods (Algebraic and Input-Output) for solving this question type, which I call Variables in the Answer Choices.
If you'd like more information on these approaches, we have some free videos:
- Variables in the Answer Choices - http://www.gmatprepnow.com/module/gmat- ... /video/933
- Tips for the Algebraic Approach - http://www.gmatprepnow.com/module/gmat- ... /video/934
- Tips for the Input-Output Approach - http://www.gmatprepnow.com/module/gmat- ... /video/935


Cheers,
Brent
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We are given that newspaper A sold for $1 and that newspaper B sold for $1.25.

Next we are are given that that p percent of the newspapers that sold were copies of newspaper A. However, we are not given the total number of copies of both newspapers sold. We can let T = the total copies of both newspapers sold. This means:

(p/100)T = copies of newspaper A sold

This also means that:

(1 – p/100)T = copies of newspaper B sold

We are finally given that r percent of the revenue came from newspaper A. We can translate this into an expression and simplify from there.


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the question mentions news papers of A and (copies) of newspapers of A , where do we count B? i got really confused with this question please help

This problem was definitely something to work on. I was able to understand it with Pikachu’s help, although, the calculations were not very clear (substitution of P in % and in Number).

So, I am going to try and put it in my own vision of explanation, using number pickings as suggested, since algebraic calculations is really long and seems very confusing to me. Please correct my mistakes.

So what we have here is:
Newspapers A sold – $1/piece
Newspaper B sold – $1.25/piece

r - % of revenue generated from A newspapers sales
p - % of A newspapers sold.

Let’s use a and b as numbers of A & B np sold.
a + b = a total number of newspapers sold.

% of something = (part/whole)*100%. => p = (a/(a+b))*100%

now r % = (Revenue of A/Revenue A + Revenue B)*100% =>
R(total) = RevenueA +RevenueB. (revenue = price * number of pieces sold)
Having price np A sold and 1$/piece and np B sold 1.25$/piece the formula is as follows:

R(total) =1$*a + 1.25$b or = a + 1.25b
r = (a/(a+1.25b))*100%

Now let’s use numbers:
20 np A sold
80 np B sold.

Total np sold = 100.
p (% of np A sold) = 20%
Total revenue R = (1$*20 + 1.25$*80) = 20 + 100 = 120$
r (% of revenue A sold) = (20$/120$)*100% = 16.7%

Now lest try and plug in the numbers. We are looking for r = 16.7%, the final result is to be multiplied to 100%. Also in numerator we will be p in % as 0.2, and denominator – we are using p – as a number – 20 (number of A newspapers sold in pieces and not in %)

(a) (100(pieces)*0.2)/(125(pieces) – 20(pieces)) = (20/105)*100 = 19% - incorrect
(b) (150*0.2)/(250 – 20) = 30/230 = 3/23 = 13% => incorrect
(c) (300*0.2)/(375 – 20) = 12/71 = 16,9 % (we are looking for 16,7%) => incorrect
(d) (400*0.2)/(500-20) = 1/6 = 16.7% - correct.
(e) (500*0.2)/(635 – 20) = 100/615 = 50/123 = 40%. => incorrect

Now let’s check it and pick other numbers:
40 np A sold
60 np B sold

Total np sold are 100. R total = 115$
p = 40%, 40 in $
r = 34.7% or ~ 35%.

(a) 100*0.4/(125 – 40) = 8/17= 47% => incorrect
(b) 150*0.4/(250 – 40) = 6/21 = 29% => incorrect
(c) 300*0.4/(375 – 40) = 24/17 ~ 36% (again close but incorrect)
(d) 400*0.4/(500 – 40) = 8/23 = 34.7% correct
(e) 500*0.4/(625 – 40) = 40/117 = 34%

Seems like D is a correct answer for both. Please correct me if I am making any errors in understanding the concept of how to approach this problem, since, the way I see it – it is a key to solving it in 2 minutes.

Hi,

Thought process: There are two ways to calculate the revenue of the newspaper A: 1. Direct revenue calculation, and 2. Using total revenue.

Let the number of the newspaper sold = 100


Revenue from A = p --- (1)

If r percent of the store’s revenues from newspaper sales was from Newspaper A => \(\frac{(125 - 0.25p)*r}{100}\) --- (2)

From (1) and (2) we have following:

\(\frac{(125 - 0.25p)*r}{100} = p \Rightarrow r = \frac{100p}{125 - 0.25p} = \frac{400p}{500 - p}\)

Thanks.

Hello Bunuel, this problem seems so confusing, even when person knows algebra very well, i think during actual exam it will be time consuming to use algebraic approach As for number plugging any tips for using this strategy? not all numbers will give correct answer, /quick solution when applying number plugging in such kind of questions, no ?

By the way i am trying to understand your algebraic approach how did you derive from this \(p=\frac{a}{a+b}*100\) this equation and how do you call this process/step ---> \(b=\frac{a}{p}*100-a=\frac{a(100-p)}{p}\) and from that how how got this one \(r=\frac{a}{a + 1.25*\frac{a(100-p)}{p}}*100\) and how call this process / step ?

thanks!

Hello generis , pushpitkc

I find this problem really hard i tried to tackle it but got into pitstop

here is what i did:

Let the price of newspapers \(A\) be \($ 1,00\)

Let the price of newspapers \(B\) be \($ 1,25\)

Let total number of newspaper \(A\) sold be\(A\)

Let total number of newspaper \(B\) sold be \(B\)

Let total number of newspapers sold be \(T\)

\(T = A+B\)

if Total Revenue is \(($1 *A ) + ($1.25 *B)\) --->

then \(R\) percent of the store’s revenues from newspaper sales was from Newspaper \(A\) --> \(\frac{R}{100}\) * \(($1 *A ) + ($1.25 *B)\)

Also, if total number of newspapers sold is \(A+B\), --->

then \(P\) percent of the newspapers that the store sold were copies of newspaper \(A\) ---> \(\frac{P}{100}\) *\((A+B)\)

ok, after the above mentioned steps I got stuck, what to do now I would appreciate explanation for dummies

have an awesome weekend

Can you please check if my approach is correct:

Assumption: I have assumed P= 50% and the total copies sold as 200.


Therefore Revenue from A= 1*100 =100 and B=1.25*100=125.
Total revenue= 225

Hence r= (100/225)* 100 =400/9.

If we plug in P=50 in all the options only D gives the answer.

I'm having a hard time with this problem.

I chose

100 units sold of A
20 units sold B

Revenue = (100)(1) + (1.25)(20) = 125
r% = 100/125
r=80

p% = 100/120
p= 83.33

Plug in numbers to get "r"

(400*83.33)/500-p

My question is, how do you know what numbers to pick to get you easy to work with numbers? Additionally, I understand the whole concept of choosing 100 and 100 or saying that you don't sell any B books, but isn't there a possibility that somehow picking the same number with "cancel" itself out?

Appreciate any response!