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Question Stats:
65% (02:11) correct
34% (01:23) wrong based on 1 sessions
If the power of an engine grows by 10% when the number of its cylinders is increased by one, which of the following is closest to the ratio of the power of a 9-cylinder engine to that of a 12-cylinder engine? (A) 0.69 (B) 0.71 (C) 0.72 (D) 0.75 (E) 0.78 OA Source: GMAT Club Tests - hardest GMAT questions OE Let X denote the power of the 9-cylinder engine and Y the power of the 12-cylinder engine. It follows from the stem that:
Y = 1.1^3X = 1.21*1.1*X = 1.331X ~= 1\frac{1}{3}X
The required ratio is \frac{X}{Y} = 1/1\frac{1}{3} = \frac{3}{4} = 0.75 My approach - Could someone validate if this approach is right?? Let P be the power of the engine.
1 cylinder results in a 10% increase in power.
Therefore 0.1P = 1 Cylinder.
Power of a 9 cylinder engine = 9 * 0.1P = 0.9P
Power of a 12 cylinder engine = 12 * 0.1P = 1.2P
Ratio of power of 9 cylinder engine to 12 cylinder engine= \frac{0.9P}{1.2P} = \frac{3}{4} = 0.75 _________________
My debrief: done-and-dusted-730-q49-v40
Last edited by sidhu4u on 28 May 2010, 23:33, edited 2 times in total.
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Manager
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hope thats a good way.... Can you post the OE? I got bogged down by trying to compound the 10%everytime. Is that not the right way to do this?
(P+0.1P)0.1P .... and so on?
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(1*1*1)/1.1*1.1*1.1 ~ 0.75
Answer D
Does this help?
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Senior Manager
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pranrasvij wrote: hope thats a good way.... Can you post the OE? I got bogged down by trying to compound the 10%everytime. Is that not the right way to do this?
(P+0.1P)0.1P .... and so on? I've posted the OE along with the OA.
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My debrief: done-and-dusted-730-q49-v40
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thanks for the OE-- that clarifies it....
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I think its D.
No. of Engines 9 10 11 12
Power(goes up by 10%) 10 11 12.1 13.31
10/13.31 = 0.75
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I solved it using the pricipal of compound intrest
A= P(1+r)^n
Assuming principal P is 1 i.e 1st year
r= rate of intrest
n= no. of years
1) for 9 years
A1=1(1+ .10)^8
2) for 12 years
A2=1(1+ .10)^12
hence ratio is A1/A2 = 1/1.10^3 ~ 1/1.33 = 100/133 = .75
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Agree with the solution by wisdompearls... D for me too 
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If the power of an engine grows by 10% when the number of its cylinders is increased by one, which of the following is closest to the ratio of the power of a 9-cylinder engine to that of a 12-cylinder engine?
(A) 0.69
(B) 0.71
(C) 0.72
(D) 0.75
(E) 0.78
I agree with everyone who chose D
A good question to pick numbers:
Let the power of 9-cylinder engine be 1,
Increases from 9 - 12 cylinder has 3 steps of power increment
Therefore, the ratio = 1/ (110/100*110/100*110/100) =
100*100*100/110*110*110 =
1000/1331=0.75
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This is a geometric sequence.
the nth term in a geometric sequence is a*( r^(n-1) )
the ratio of 9th to 11th is ar^8/ar^11 = 1/r*r*r = 1/1.331 = 0.75
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since power increase by 10% => common ratio = 1.1 let the power of a 9-engine = p (1st term of a GP series) Power of a 12(4th term) engine = p(1.1)^3 9- engine / 12-engine = p/(p)1.1^3 = 1/1.331 0.751 The closest is D (0.75)
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D too.
by choosing the power of 9 cylinders is 1.
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hothihongcam wrote: D too.
by choosing the power of 9 cylinders is 1. Can you emphasize on your method...with particular reference to the highlighted parts. Thanks.
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Its C.
The ratio is 1: 1.1 to the power 3. Close to 0.75!
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sidhu4u wrote: My approach - Could someone validate if this approach is right?? Let P be the power of the engine.
1 cylinder results in a 10% increase in power.
Therefore 0.1P = 1 Cylinder.
Power of a 9 cylinder engine = 9 * 0.1P = 0.9P
Power of a 12 cylinder engine = 12 * 0.1P = 1.2P
Ratio of power of 9 cylinder engine to 12 cylinder engine= \frac{0.9P}{1.2P} = \frac{3}{4} = 0.75 I think you got lucky in this case. You should have to do it exponentially. I took a long approach, assuming the power of a 9 cylinder engine is 100: 10 cylinder power: 110 11: 121 12: 133 100/133 was not something I could compute quickly, so I picked .71 first because it was the easiest calculation, then went to .75 and found that it was correct.
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I think the simplest way is: an increase in 10% simply means a1.1 increase so
9*1.1=9.9
12*1.1=13.2
Ratio of the two is 9.9/13.2=0.75
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I used the compound formaule approach as the power compounds.... so....y=x(1+10/100)^3=x(11/10)^3 The question is what is x/y ? or what is (10/11)^3... I agree that the ANS. is (D) if you actually solve putting the values... But what I did was [(11-1)/11]^3=(1-.09)^3=(.91)^3 Now I approx. this to (.9)^3 giving .729 So I chose (C) that was wrong  Do these types of Questions where you have to actually solve the values rather than apply logic really come in GMAT ?
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My approach was,
We are asked to find the ration of power with 9 cylinder to that of 12 cylinder engine.
Given: addition of 1 cylinder will increase the power by 10%. Say if power of engine is 1, then adding 1 cylinder to it makes the power as 1.1
=P(9 Cylinders)/ P(12 Cylinders)
=P(9 C)/ (P(9 C) and P(3 C))
= 1/ P(3 C)
= 1/ (1.1* 1.1 * 1.1) = 1/ 1.33 = 1/ 1.3 (approx)
= .75 (approx) Simplistic, calculation will give nearest answer (100/13 is 7.6)
Ans: D
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+1 for D.. Using the approx technique..1/13 is 0.77 and 1/14 is 0.71.. Now we have 1/13.33.. it should be more towards 0.75 rather than 0.72...
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since ratio of power for 9 and 12 cylinders is asked we dont have to really calculate the p9 and p12 as we know p/c = p9/c9 = p12/c12
since we know c9=9 cylinders and c12=12 cylinders
so after substituting the value p9/p12=9/12=0.75
so answer is D
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