If (3^x) - 3^(x-1) = 162, then x(x-1) =
A) 12
B) 16
C) 20
D) 30
E) 81
Answer is C, but exactly how?
GMATPrep Problem
This topic has expert replies
-
- Junior | Next Rank: 30 Posts
- Posts: 27
- Joined: Wed Feb 13, 2008 10:34 am
- Location: India
- Thanked: 3 times
Since, (3^x) - 3^(x-1) = 162
 (3^(x-1))(3 -1) = 162
 (3^(x-1))2 = 162
 (3^(x-1)) = 162/2 = 81 = 3^4
 x-1 = 4
 x = 5
Therefore, x(x-1) = 4*5 = 20…………… your answer
 (3^(x-1))(3 -1) = 162
 (3^(x-1))2 = 162
 (3^(x-1)) = 162/2 = 81 = 3^4
 x-1 = 4
 x = 5
Therefore, x(x-1) = 4*5 = 20…………… your answer
Let the Game begin
-
- Senior | Next Rank: 100 Posts
- Posts: 50
- Joined: Tue Apr 18, 2006 5:41 pm
- Thanked: 2 times
- Followed by:1 members
-
- Legendary Member
- Posts: 541
- Joined: Thu May 31, 2007 6:44 pm
- Location: UK
- Thanked: 21 times
- Followed by:3 members
- GMAT Score:680
the above equation can also be written as:musicdaemon wrote:Since, (3^x) - 3^(x-1) = 162
3x3^(x-1) - 3^(x-1) = 162
taking 3^(x-1) common
we get:
3^(x-1) [3-1] = 162
musicdaemon wrote:  (3^(x-1))(3 -1) = 162
 (3^(x-1))2 = 162
 (3^(x-1)) = 162/2 = 81 = 3^4
 x-1 = 4
 x = 5
Therefore, x(x-1) = 4*5 = 20…………… your answer
-
- Senior | Next Rank: 100 Posts
- Posts: 36
- Joined: Tue Jul 20, 2010 7:42 pm
- Thanked: 1 times
- Followed by:1 members
I am very confused
When factoring out the common factor how are you getting 3x3^x-1 - 3^x-1=162
Isnt the original question 3^x-3^x-1=162 So wouldn't the common factor be 3x can someone please explain step by step.
Thank you,
RB
When factoring out the common factor how are you getting 3x3^x-1 - 3^x-1=162
Isnt the original question 3^x-3^x-1=162 So wouldn't the common factor be 3x can someone please explain step by step.
Thank you,
RB
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
We can plug in the answers, which represent the value of x(x-1).adi wrote:If (3^x) - 3^(x-1) = 162, then x(x-1) =
A) 12
B) 16
C) 20
D) 30
E) 81
Answer is C, but exactly how?
Answer choice C: x(x-1) = 20.
If x(x-1) = 20, then x = 5, since 5*(5-1) = 5*4 = 20.
Plugging x=5 into 3^x - 3^(x-1) = 162, we get:
3� - 3^(5-1) = 162
3� - 3� = 2 * 3�
3�(3-1) = 2 * 3�
3� * 2 = 2 * 3�. Success!
The correct answer is C.
Alternatively, the numbers are small enough that we could plug in the answers and expand the exponents:
Answer choice C: x(x-1) = 20.
If x(x-1) = 20, then x = 5, since 5*(5-1) = 5*4 = 20.
Plugging x=5 into 3^x - 3^(x-1) = 162, we get:
3� - 3^(5-1) = 162
3� - 3� = 162
243 - 81 = 162. Success!
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
- smackmartine
- Legendary Member
- Posts: 516
- Joined: Fri Jul 31, 2009 3:22 pm
- Thanked: 112 times
- Followed by:13 members
162 can be written as 2 * 3^4
& 3^(x-1) can be written as (3^x)/3
now given, 3^x - 3^(x-1) = 2 * 3^4
=> 3^x - (3^x)/3 = 2 * 3^4
Take 3^x common ---> 3^x[ 1-(1/3)] = 2* 3^4
3^x [2/3]== 2* 3^4
cancelling 2 on both sides (3^x)/3 = 3^4
Taking denominator 3 on other side we get 3^x = 3^5
=> x=5 (as bases are same)
now x (x-1) is 5 (5-1) = 5*4 = 20 (C)
Hope it helps!.
& 3^(x-1) can be written as (3^x)/3
now given, 3^x - 3^(x-1) = 2 * 3^4
=> 3^x - (3^x)/3 = 2 * 3^4
Take 3^x common ---> 3^x[ 1-(1/3)] = 2* 3^4
3^x [2/3]== 2* 3^4
cancelling 2 on both sides (3^x)/3 = 3^4
Taking denominator 3 on other side we get 3^x = 3^5
=> x=5 (as bases are same)
now x (x-1) is 5 (5-1) = 5*4 = 20 (C)
Hope it helps!.
- Stuart@KaplanGMAT
- GMAT Instructor
- Posts: 3225
- Joined: Tue Jan 08, 2008 2:40 pm
- Location: Toronto
- Thanked: 1710 times
- Followed by:614 members
- GMAT Score:800
As a follow-up to Mitch's post (backsolving is a great way to solve this question), we can use common sense and a bit of arithmetic to quickly eliminate two of the five choices, making backsolving even more viable.adi wrote:If (3^x) - 3^(x-1) = 162, then x(x-1) =
A) 12
B) 16
C) 20
D) 30
E) 81
Answer is C, but exactly how?
We know that the correct answer is the product of x and (x-1). Since this is the GMAT, it's reasonable to assume that x is going to be an interger (otherwise the problem would be well beyond the scope of the exam).
So, which answers can be written as the product of two consecutive integers?
A) 12=3*4... keep it
B) 3*4 = 12; 4*5=20... no way to write 16 as the product of 2 consecutive integers - eliminate B.
C) 20 = 4*5... keep it
D) 30 = 5*6... keep it
E) 8*9 = 72; 9*10 = 90... no way to write 81 as the product of 2 consecutive integers - eliminate E.
Since we only have 3 possible choices, we know that the worst case scenario is that we have to plug 2 of them in (if neither of those work, then the third choice is correct by default).
Stuart Kovinsky | Kaplan GMAT Faculty | Toronto
Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
-
- Master | Next Rank: 500 Posts
- Posts: 158
- Joined: Mon Nov 02, 2009 5:49 pm
- Thanked: 2 times
- Followed by:3 members
1. 3^(x) - 3^(x-1) = 162
2. Factor out 3^x ----> 3^(x) {1 - 3^(-1)} = 162
3. Further simplify: 3^(x) {1 - (1/3) ] = 162
4. 3^(x) (2/3) = 162
5. 3^(x) = 243 ---> x = 5
6. x (x-1) = 5 (5 -1)----> 20
OA = C
2. Factor out 3^x ----> 3^(x) {1 - 3^(-1)} = 162
3. Further simplify: 3^(x) {1 - (1/3) ] = 162
4. 3^(x) (2/3) = 162
5. 3^(x) = 243 ---> x = 5
6. x (x-1) = 5 (5 -1)----> 20
OA = C
Our collective understanding of the GMAT grows through research, contribution, and teamwork. If you found a problem or comment challenging, helpful, or encouraging, please consider hitting the THANKS button!