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kaplan horses racing

by resilient » Tue Mar 18, 2008 10:15 am
Hi,

I wanted to give you a certain example of a constantly annoying type of mistake I am making. The solution from Kaplan is a bit longer but I answered through straight logic.

Question: Two horses begin running on an oval course at the same time. One runs each lap in 9 minutes; the other takes 12 minutes to run the each lap. How many minutes after the start will the faster horse have one lap lead?

a.36
b.12
c.9
d.4
e.3

my approach: on each lap the faster horse gains 1/4 lap and therefor on 4 laps he will have one lap on the slower horse. I answered as 4 minutes. I forgot to further synthesize and multiply the 4 laps by his time of 9 minutes. THerefore correct answer is 4x9=36.

I am being hasty and ruining my scores even though I understand the math. Any words on how to stop this?
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by xilef » Tue Mar 18, 2008 10:50 am
I was also about to answer 4 because of the 4 laps but then glanced over the question and noticed that the question was asking for minutes. So take another glance at the question before you answer.
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by Stuart@KaplanGMAT » Tue Mar 18, 2008 2:01 pm
Step 4 of the Kaplan method for problem solving: double check the question!

The mistake you made is the most psychologically damaging and demoralizing type of mistake on the GMAT - you did 95% of the work to get the question right, but you didn't answer what the question was actually asking. GMAT questions are designed to punish people who aren't careful.

For example, almost every question that asks you to solve for 2x, or 3x, or 1/2x will have the value of x as one of the choices. GMAC knows that test takers get really excited that they've solved for x and often forget that the question was asking for something different.

It's definitely worth spending an extra 5 seconds on each question to 100% avoid that type of mistake. Before clicking the bubble on the screen, quickly re-read the actual question and make sure that you're answering exactly what the question demands.
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by Jimat » Tue Mar 18, 2008 2:51 pm
Is correct answer a.36???

Here is how I solved it:
H1: Horse which runs the lap in 9 min
L1: Distance ran by the H1.
V1: Speed of H1.
H2: Horse which runs the lap in 12 min
L2: Distance ran by the H2.
V2: Speed of H2.

Lets say that the distance of a lap is a multiple of 9 and 12, so I chose 36. So V1 is 36/9 = 4 and V2 is 36/12 =3.

The question asks for when H1 will be 36 (1 lap) ahead H2. Then the variable T (time) will be the same for the 2 horses and the distance of the faster horse (H1) will be 36 more than that of H2. T = Distance/Speed

L1 = 36+L2.

(L2+36)/V1 = L/V2, then L2 = 108 => T = 36.

I hope this helps.

Regards,
Jimat
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by xilef » Tue Mar 18, 2008 3:18 pm
Jimat:

Thanks for the solution but the original question asks for tips that will help avoid careless mistakes and answer the question that is being asked; I guess your misinterpretation of the posted question is relevant after all :)
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