November 20, 2018 November 20, 2018 09:00 AM PST 10:00 AM PST The reward for signing up with the registration form and attending the chat is: 6 free examPAL quizzes to practice your new skills after the chat. November 20, 2018 November 20, 2018 06:00 PM EST 07:00 PM EST What people who reach the high 700's do differently? We're going to share insights, tips and strategies from data we collected on over 50,000 students who used examPAL.
Author 
Message 
TAGS:

Hide Tags

Manager
Joined: 02 Dec 2012
Posts: 177

Is 5^k less than 1,000?
[#permalink]
Show Tags
26 Dec 2012, 03:41
Question Stats:
70% (01:49) correct 30% (02:01) wrong based on 1262 sessions
HideShow timer Statistics
Is 5^k less than 1,000? (1) \(5^{(k+1)} > 3,000\) (2) \(5^{(k1)} = 5^k  500\)
Official Answer and Stats are available only to registered users. Register/ Login.




Math Expert
Joined: 02 Sep 2009
Posts: 50624

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
26 Dec 2012, 03:42
Is 5^k less than 1,000?Is \(5^k<1,000\)? (1) 5^(k+1) > 3,000 > \(5^k>600\) > if \(k=4\) then the answer is YES: since \(600<(5^4=625)<1,000\) but if \(k=10\), for example, then the answer is NO. Not sufficient. (2) 5^(k1) = 5^k  500 > we can solve for k and get the single numerical value of it, hence this statement is sufficient. Just to illustrate: \(5^k5^{k1}=500\) > factor out \(5^{k1}\): \(5^{k1}(51)=500\) > \(5^{k1}=125\) > \(k1=3\) > \(k=4\). Sufficient. Answer: B. Hope it's clear.
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics




Retired Moderator
Joined: 29 Oct 2013
Posts: 265
Concentration: Finance
GPA: 3.7
WE: Corporate Finance (Retail Banking)

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
23 May 2014, 13:08
The biggest takeaway here should be that we don't need to solve the St2. The moment you start solving St2, you have fallen for GMAT's classic time waster trap. See below. Statement1: As 5^(k+1) > 3,000 > k>4 and hence insufficient Statement2: We dont need to solve the equation. Since this is an EQUATION (and not an inequality) with one variable, we will get the exact value of k and we will be able to answer the question one way or the other. SUFFICIENT.
_________________
Please contact me for super inexpensive quality private tutoring
My journey V46 and 750 > http://gmatclub.com/forum/myjourneyto46onverbal750overall171722.html#p1367876




Intern
Joined: 07 Oct 2012
Posts: 8

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
06 Apr 2013, 22:12
Walkabout wrote: Is 5^k less than 1,000?
(1) 5^(k+1) > 3,000
(2) 5^(k1) = 5^k  500 I would to ask why is this wrong (1) 5^(k+1) > 3000 5^(k+1) > 5^5 Hence, k+1= 5 , k =4 If so, 5^4 is less than 1000. The answer should b sufficient for (1).



Math Expert
Joined: 02 Sep 2009
Posts: 50624

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
07 Apr 2013, 22:03
LMKong wrote: Walkabout wrote: Is 5^k less than 1,000?
(1) 5^(k+1) > 3,000
(2) 5^(k1) = 5^k  500 I would to ask why is this wrong (1) 5^(k+1) > 3000 5^(k+1) > 5^5 Hence, k+1= 5 , k =4 If so, 5^4 is less than 1000. The answer should b sufficient for (1). First of all 5^5=3,125>3,000, thus 5^(k+1) > 3000 does NOT necessarily mean that 5^(k+1) > 5^5. Next, even if we had 5^(k+1) > 5^5 it still does not mean that k+1=5. It means that k+1>5 > k>4. Hope it's clear.
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Manager
Joined: 09 Apr 2013
Posts: 113
Location: India
WE: Supply Chain Management (Consulting)

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
21 Nov 2013, 10:05
Statement(1) : 5^(k+1) > 3000 The above inequality can be reduced to 5^k > 600. From this, we clearly know few possible values for k i.e., 4,5,6,.. Substituting these values in the inequality given in the question gives away both yes and no answers. k = 4, 5^(41) < 1000 k = 5, 5^(51) < 1000 k = 6, 5^(61) > 1000 Hence statement(1) is not sufficient. Statement(2): 5^(k1) = 5^k  500 Reducing the above inequality, 4/5 * 5^k = 500 So 5^k = 625 = 5^4. Clearly k = 4 and the original inequality is satisfied: 5^4 < 1000. Hence statement(2) is sufficient. Answer: B
_________________
+1 KUDOS is the best way to say thanks
"Pay attention to every detail"



EMPOWERgmat Instructor
Status: GMAT Assassin/CoFounder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 12877
Location: United States (CA)

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
16 Jan 2015, 12:08
Hi All, Right from the beginning, there are 2 things that you should note about this DS prompt: 1) At NO POINT does it state that K has to be an integer. 2) It's clearly based on exponents, so some exponent rules/patterns MUST be involved. We're asked if 5^K is < 1,000. This is a YES/NO question. Fact 1: 5^(K+1) > 3,000 In this Fact, notice how the exponent (K+1) differs from the exponent in the question (K). There's an exponent rule that accounts for this difference. As an example, consider... 5^2 = 25 5^3 = 125 Notice how 5^3 is "5 times" greater than 5^2? This difference occurs because the base is 5 and we're increasing the exponent by 1. It can also be used in reverse.... 5^3/5^2 = 5^(32) = 5^1 = 5 This is a standard rule about "dividing" exponents with the same base > we SUBTRACT the exponents. With Fact 1, we're dealing with 5^(K+1) and the question is dealing with 5^K. This means that DIVIDING 5^(K+1) by 5 will give us 5^K: 5^(K+1)/5^1 = 5^(K+11) = 5^K. This is all meant to say that we can DIVIDE both sides of this inequality by 5, which gives us... 5^(K+1) > 3,000 5^K > 600 IF.... 5^K = 601 then the answer to the question is YES 5^K = 1,001 then the answer to the question is NO Fact 1 is INSUFFICIENT Fact 2: 5^(K1) = 5^K  500 This is a 1 variable, 1 equation "system", so we CAN solve it (and there will only be 1 answer). Even if you did not know that, it's still easy enough to get to the solution.... Since most Test Takers are better at basic multiplication than they are at manipulating higherlevel exponents, here's how you can "brute force" the solution: 5^1 = 5 5^2 = 25 5^3 = 125 5^4 = 625 5^5 = 3125 Find two consecutive powers of 5 that differ by 500 and you have the solution to the above equation. 5^4  5^3 = 625  125 = 500 Fact 2 is SUFFICIENT. Final Answer: GMAT assassins aren't born, they're made, Rich
_________________
760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com
Rich Cohen
CoFounder & GMAT Assassin
Special Offer: Save $75 + GMAT Club Tests Free
Official GMAT Exam Packs + 70 Pt. Improvement Guarantee www.empowergmat.com/
*****Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!*****



Current Student
Joined: 12 Aug 2015
Posts: 2633

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
08 Mar 2016, 21:18



CEO
Joined: 11 Sep 2015
Posts: 3122
Location: Canada

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
13 Jan 2018, 11:18
Walkabout wrote: Is 5^k less than 1,000?
(1) \(5^{(k+1)} > 3,000\)
(2) \(5^{(k1)} = 5^k  500\) Target question: Is 5^k less than 1000? Statement 1: 5^(k+1) > 3000First notice that 5^(k+1) = (5^k)(5^1) So, we can take 5^(k+1) > 3000 and divide both sides by 5 to get: 5^k > 600 There are several possible cases to consider. Here are two: case a: 5^k = 601, in which case 5^k is less than 1000. case b: 5^k = 1001, in which case 5^k is not less than 1000. Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT. Statement 2: 5^(k1) = 5^k  500IMPORTANT: Notice that we're given an EQUATION, which means we can solve the equation to find the definitive value of k. If we can find the value of k, then we can instantly tell whether or not 5^k is less than 1000. So, it SEEMS that we can conclude that statement 2 is sufficient WITHOUT performing any calculations. HOWEVER, if it's the case that the equation yields 2 possible values of k, then it may be the case that one value of k is such that 5^k is less than 1000, and the other value of k is such that 5^k is greater than 1000. So, at this point, we need only determine whether or not the equation will yield 1 or 2 values of k. Rearrange to get the k's on one side: (5^k)  5^(k1) = 500 Factor the left side: 5^(k1)[5  1] = 500 Simplify: 5^(k1)[4] = 500 STOP!!At this point, we can see that this equation will yield only one value of k. So, IF WE WERE to solve the equation for k, we would definitely be able to determine whether or not 5^k is less than 1000. Since we can answer the target question with certainty, statement 2 is SUFFICIENT. Answer = BCheers, Brent
_________________
Test confidently with gmatprepnow.com



Manager
Joined: 10 Apr 2018
Posts: 181

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
13 Aug 2018, 12:05
Hi Brent GMATPrepNow and Bunuel, Will this approach work 5^k< (5^3* 2^3) So if K<= 4 then we can get answer for our question. 1) 5^(k+1)>3,000 5^(k+1)>3 * 5^3 * 2^3 so k+1>3 so k> 2 So insufficient. 5^(k−1)=5^k−500 5^k−5^(k−1)=500 5^k{ 11/5}= 500 5^k{4/5}=500 4*5^k= 2500 4*5^k=5^4 * 4 so k=4 So sufficient .



Intern
Joined: 10 Oct 2017
Posts: 6

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
06 Nov 2018, 05:35
Walkabout wrote: Is 5^k less than 1,000?
(1) \(5^{(k+1)} > 3,000\)
(2) \(5^{(k1)} = 5^k  500\) Correct me if my reasoning is flawed. first of all the question asks is 5^k<1000, in other words is k<=4, since 5^4= 625 and 5^5=3125. Then we have (1) 5^(k+1)>3000, since 5^5=625, we have k>=4, NOT k>4, because when k=4, the inequality holds true. So, here we have one scenario that k=4, because in the original question we know that k<=4 and here we have k>=4, therefore only k=4 holds true. Therefore, D should be the answer.



EMPOWERgmat Instructor
Status: GMAT Assassin/CoFounder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 12877
Location: United States (CA)

Re: Is 5^k less than 1,000?
[#permalink]
Show Tags
06 Nov 2018, 11:55
Hi hovhannesmkrtchyan, When dealing with Quant questions, you have to be careful about distinguishing between what you KNOW and what DON'T KNOW. This question does NOT tells us that K is an integer, so you have to be open to the idea that it might not be a whole number. By extension, if K is a NONINTEGER, then 5^K will not actually be a multiple of 5. If you read the other explanations in this thread, you'll see why Fact 1 is insufficient. GMAT assassins aren't born, they're made, Rich
_________________
760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com
Rich Cohen
CoFounder & GMAT Assassin
Special Offer: Save $75 + GMAT Club Tests Free
Official GMAT Exam Packs + 70 Pt. Improvement Guarantee www.empowergmat.com/
*****Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!*****




Re: Is 5^k less than 1,000? &nbs
[#permalink]
06 Nov 2018, 11:55






