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# A reduction in the price of petrol by 10% enables a motorist

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Director
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A reduction in the price of petrol by 10% enables a motorist [#permalink]  17 Feb 2012, 20:29
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Question Stats:

66% (02:23) correct 34% (01:48) wrong based on 177 sessions
A reduction in the price of petrol by 10% enables a motorist to buy 5 gallons more for $180. Find the original price of petrol? A.$11
B. $5 C.$45
D. $400 E.$4
[Reveal] Spoiler: OA

Last edited by LM on 17 Feb 2012, 20:44, edited 1 time in total.
Senior Manager
Status: May The Force Be With Me (D-DAY 15 May 2012)
Joined: 06 Jan 2012
Posts: 292
Location: India
Concentration: General Management, Entrepreneurship
Followers: 1

Kudos [?]: 68 [0], given: 16

Re: Percentages-PS [#permalink]  17 Feb 2012, 20:41
I think this is the way to solve it not really sure

Let X be no of litres of petrol & n be the price per litre

Eqn 1 : x*n = 180

Eqn 2 : (x+5) * (0.9n) = 180

Simplifying eqn 2

0.9xn + 4.5n = 180

Substituting eqn 1 in eqn 2

0.9 ( 180) + 4.5n = 180

n = 18/4.5 = 4

Hence E

PS : Thanxs LM for pointing out my error
_________________

Giving +1 kudos is a better way of saying 'Thank You'.

Last edited by boomtangboy on 17 Feb 2012, 20:54, edited 1 time in total.
Director
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Re: Percentages-PS [#permalink]  17 Feb 2012, 20:44
boomtangboy wrote:
I think this is the way to solve it not really sure

$100 - > 100 Litre$ 90 -> 105 Litre
$180 - > ? => 180 * 105 /90 => 210 180/210 => 6/7 =>$ 0.14 / Litre

=> 210 Litres

Director
Joined: 03 Sep 2006
Posts: 893
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Kudos [?]: 91 [0], given: 33

Re: Percentages-PS [#permalink]  17 Feb 2012, 21:00
boomtangboy wrote:
I think this is the way to solve it not really sure

Let X be no of litres of petrol & n be the price per litre

Eqn 1 : x*n = 180

Eqn 2 : (x+5) * (0.9n) = 180

Simplifying eqn 2

0.9xn + 4.5n = 180

Substituting eqn 1 in eqn 2

0.9 ( 180) + 4.5n = 180

n = 18/4.5 = 4

Hence E

PS : Thanxs LM for pointing out my error

Correct and systematic approach. I was just wondering whether it can be solved within a minute or so without making efforts of too much calculations. Any fast method to solve percentages in such kind of questions?
Math Expert
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Re: Percentages-PS [#permalink]  17 Feb 2012, 21:16
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Expert's post
LM wrote:
A reduction in the price of petrol by 10% enables a motorist to buy 5 gallons more for $180. Find the original price of petrol? A.$11
B. $5 C.$45
D. $400 E.$4

Price decreased by 10%, so 9/10 times, which means that original gallons bought increased 10/9 times. Since this increase equals to 5 gallons then 45 gallons were bought originally (45*10/9=50 --> increase 5 gallons). Hence original price was 180/45=$4. Answer: E. P.S. One can also use backsolving to get the answer. Hope it's clear. _________________ Director Joined: 03 Sep 2006 Posts: 893 Followers: 6 Kudos [?]: 91 [0], given: 33 Re: Percentages-PS [#permalink] 17 Feb 2012, 22:12 Bunuel wrote: LM wrote: A reduction in the price of petrol by 10% enables a motorist to buy 5 gallons more for$180. Find the original price of petrol?

A. $11 B.$5
C. $45 D.$400
E. $4 Price decreased by 10%, so 9/10 times, which means that original gallons bought increased 10/9 times. Since this increase equals to 5 gallons then 45 gallons were bought originally (45*10/9=50 --> increase 5 gallons). Hence original price was 180/45=$4.

P.S. One can also use backsolving to get the answer.

Hope it's clear.

Thanks a lot! How did you quickly arrive at this short method (which I was looking to!) 9/10 decrease will lead to increase of 10/9. This seems to be the key and short cut! Little more explanation or any hint or link within posts would be helpful. Awesome approach!
Math Expert
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Posts: 18789
Followers: 3259

Kudos [?]: 22575 [4] , given: 2631

Re: Percentages-PS [#permalink]  17 Feb 2012, 22:29
4
KUDOS
Expert's post
LM wrote:
Thanks a lot! How did you quickly arrive at this short method (which I was looking to!) 9/10 decrease will lead to increase of 10/9. This seems to be the key and short cut! Little more explanation or any hint or link within posts would be helpful. Awesome approach!

Consider this: original price of an item is $10, decrease the price by 50%: new price$5 ($10*1/2=$5). Now, for some fixed amount of money, say for $100, you'll be able to buy twice as many items for$5 (100/$5=20) than for$10 (100/$10=10): reduction in price (1/2) reciprocal of increase in quantity (2) (or simply for half of the price you'll buy twice as many in quantity). The same for the original question. Hope it helps. _________________ Manager Joined: 27 May 2012 Posts: 213 Followers: 0 Kudos [?]: 46 [1] , given: 105 Re: A reduction in the price of petrol by 10% enables a motorist [#permalink] 04 Sep 2012, 07:16 1 This post received KUDOS LM wrote: A reduction in the price of petrol by 10% enables a motorist to buy 5 gallons more for$180. Find the original price of petrol?

A. $11 B.$5
C. $45 D.$400
E. $4 initially he got x gallons for$180 so 1 gallon = \frac{180}{x}-- old price per gallon

after reduction of price
he got 5 gallons more , x+5 gallons for $180 so 1 gallon = \frac{180}{x+5}----new price per gallon now old price decreased by 10%, so 90% of old price = new price (\frac{90}{100})(\frac{180}{x}) = \frac{180}{x+5} --> simplifying \frac{162}{x} = \frac{180}{x+5} now solving for x we get x= 45 ,so initially he got 45 gallons for 180 hence price of 1gallon \frac{180}{45} = 4 _________________ - Stne Manager Joined: 28 Dec 2012 Posts: 50 Schools: LBS '14 Followers: 1 Kudos [?]: 15 [2] , given: 55 Re: A reduction in the price of petrol by 10% enables a motorist [#permalink] 05 Jan 2013, 02:25 2 This post received KUDOS LM wrote: A reduction in the price of petrol by 10% enables a motorist to buy 5 gallons more for$180. Find the original price of petrol?

A. $11 B.$5
C. $45 D.$400
E. $4 Here is a short-cut: Since price decreased by 10% ( i e 1/10) so for same 180$,
quantity increase will be 11.11% (i e 1/9).
So the increase in quantity will be 1/9 of original which is given to be 5.
So original quantity was 5*9 = 45. Thus original price was 180$/45 = 4$.

Concept explanation:

When ever there are problems involving change (like increase decrease) the numerator is either added or substracted from the denominator to change base. Ex. SP = 600$. Profit % = 20%. Now, Profit in$ ? Profit % is calculated on CP.
Profit% = (P*100/CP) but wee are given SP. SO we change the base from CP to SP. 20% of CP = 1/5 of CP = 1/6 of SP = 1/6 of 600 = 100 $. We have added numerator to denominator because new base required is SP, which is larger in case of profit. If it were loss, we would have subtracted. (P/CP )*100% is on base CP (denominator). which can be base changed to SP. SP = P+CP. thus, Profit % can also be shown like (P/(P+CP))*100% I will be posting so more examples soon... hope this helps and add value to this wonderfull site!!! Some kudos please.. I need 50.. am working hard to get them!!! Intern Joined: 08 Jan 2013 Posts: 4 Followers: 0 Kudos [?]: 3 [1] , given: 1 Re: A reduction in the price of petrol by 10% enables a motorist [#permalink] 10 Jan 2013, 11:54 1 This post received KUDOS I have a slightly different approach where we can directly arrive at the original unit price. Let x be the unit price. We know that we get 5 gallons more after the discount. So the difference is 5. This difference can be expressed as below 180/.9x minus 180/x = 5 where 180/.9x is the no. of gallons we get after discount and 180/x is the no. of gallons we get before discount solving for x gives you 180/45 = 4 Hope this approach helps. thanks, Mohan Senior Manager Joined: 23 Oct 2010 Posts: 384 Location: Azerbaijan Concentration: Finance Schools: HEC '15 (A) GMAT 1: 690 Q47 V38 Followers: 11 Kudos [?]: 126 [2] , given: 73 Re: A reduction in the price of petrol by 10% enables a motorist [#permalink] 27 Jan 2013, 03:55 2 This post received KUDOS NP=180 0.9P(N+5)=180 0.9P(N+5)=NP N=45 P=180/45=4 _________________ Happy are those who dream dreams and are ready to pay the price to make them come true Intern Joined: 09 Oct 2013 Posts: 5 Followers: 0 Kudos [?]: 1 [0], given: 1 Re: A reduction in the price of petrol by 10% enables a motorist [#permalink] 21 Oct 2013, 10:58 LM wrote: A reduction in the price of petrol by 10% enables a motorist to buy 5 gallons more for$180. Find the original price of petrol?

A. $11 B.$5
C. $45 D.$400
E. $4 The question never explicitly states that .9P(X+5) = PX... SVP Joined: 06 Sep 2013 Posts: 1629 Location: United States Concentration: Finance GMAT 1: 710 Q48 V39 WE: Corporate Finance (Investment Banking) Followers: 10 Kudos [?]: 135 [0], given: 254 Re: A reduction in the price of petrol by 10% enables a motorist [#permalink] 03 Jan 2014, 06:45 LM wrote: A reduction in the price of petrol by 10% enables a motorist to buy 5 gallons more for$180. Find the original price of petrol?

A. $11 B.$5
C. $45 D.$400
E. \$4

This is more of a word problem, but anyways

One gets (0.9P)(Q+5)=180

Now, once we have this it is better to backsolve. Obviously, D is total out.
I'll start with E since answer choices are not in order and I'm guessing GMAT expects me to work in order

180/4 = 45

Replacing in original equaiton one gets (9/10*4)(50) = 180

It matches so answer is E

Hope it helps
Gimme Kuddos if it does ok?

Cheers!
J
Re: A reduction in the price of petrol by 10% enables a motorist   [#permalink] 03 Jan 2014, 06:45
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