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Arithmetic Prob

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Arithmetic Prob

by selango » Wed May 26, 2010 1:56 am
OG 12

As X increases from 165 to 166,which of the following must increase?

i)2x-5

ii)1-1/x

iii)1/(x^2-x)


A) i only

B)iii only

C)i and ii

d)i and iii

e) ii and iii
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by liferocks » Wed May 26, 2010 2:02 am
since x is an integer and increases
2x-5 will increase

1/x will decrease and hence 1-1/x increase

x(x-1) increase..hence 1/(x^2-x) decrease

so ans C
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by thephoenix » Wed May 26, 2010 2:09 am
1) 2x-5 as x increases from 2x will inc for the given condition and therefore the value for 2x-5 will also inc
2) for the given condition 1/x decreases wrt to increasing value of x and therefore the value for 1-1/x will also inc
3)1/x(x-1) x(x-1) increases and the corresponding value of the exp dec

so only 1 and 2 are increasing C
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by frank1 » Wed May 26, 2010 2:38 am
Time 1:51

IMO C

I think what is being test here is
people might start with x=165 and 166 and all(trap)
which will take lot of time...
rather than that...
if we do x=1...2...3
then it will be possible to solve it,with in 2 minutes....
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by selango » Wed May 26, 2010 4:21 am
Can we follow this method?

Instead of solving with the numbers given in prob,can we plug in another nums(like here 1...2...3) and solve?


Ur suggestions please...
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by Patrick_GMATFix » Wed May 26, 2010 1:31 pm
@Selango,

Yes, you can plug in positive integers greater than 1. Generally when a question asks you how certain numbers behave, you can plug in other numbers that have similar behaviors. Along the number line, numbers can be divided into the following groups: {-infinity to -1} {-1 to 0} {0 to 1} and {1 to +infinity}

Instead of dealing with decimals between 165 and 166, you could deal with 2 or 3.

PS. The technique above wouldn't apply if the question is about even/odd of course, since that property divides numbers into different groupings.

This is OG12 # 176. A couple of ways to solve are attached.

-Patrick
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OG12-PS176.pdf
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