If 3^x - 3^(x-1) = 162, then x(x - 1) =

Question

If 3^x - 3^{(x-1)} = 162 , then x(x - 1) = A. 12 B. 16 C. 20 D. 30 E. 81 Picture 8.png

IanStewart · Accepted Answer

Just as you'd factor out the term with the smaller power if you saw something like  x^8 - x^7  (you'd factor out  x^7 ) you can factor out the term with the smaller power here:

\begin{align*}
3^x - 3^{x-1} &= 162 \
3^{x-1}(3 - 1) &= 162 \
3^{x-1} &= 81 \
x - 1 &= 4
\end{align*}

So x = 5, and x*(x-1) = 20.

sashiim20 · Answer

3^x - 3^{(x-1)} = 162 3^x - (3^x*3^{-1}) = 162 3^x - (\frac{3^x}{3^{1}}) = 3^4*2 \frac{3*3^x - 3^x}{3} = 3^4*2 \frac{3^x(3 - 1)}{3} = 3^4*2 3^x(3 - 1) = 3^4*2*3 3^x(2) = 3^5*2 3^x = 3^5 x = 5 x (x-1) = 5(5-1) = 5*4 = 20 Answer (C)... _________________ Please Press "+1 Kudos" to appreciate.

piyatiwari · Answer

For me, fastest method was plugging in the answer choices.

From the answer choices, only 12, 20 and 30 satisfy x(x-1).

Among these only 20 i.e. x = 5 satisfies : 3^x - 3^(x-1) = 162

Hence answer is C.

iamgame · Answer

3^x - 3^x-1=162
3^x =162
3^x =162
3^x=81*3
=>x=5
x(x-1) =5*4 = 20
Hence C

EMPOWERgmatRichC · Answer

Hi All,

If you're not an expert at tougher exponent rule questions such as this, you can sometimes get to the answer with a bit of "brute force", even without knowing the exact exponent rules involved in the prompt. Here's how:

This question involves 3 raised to different "powers"; you can calculate them rather easily...

3^1 = 3
3^2 = 9
3^3 = 27
3^4 = 81
3^5 = 243

3^X and 3^(X-1) are consecutive powers of 3.
We're told that 3^X - 3^(X-1) = 162, so we just need to find 2 consecutive multiples of 3 that differ by 162.

Notice how...
3^5 - 3^4 = 
243 - 81 = 
162

This is EXACTLY what we're looking for.

X = 5
(X-1) = 4

So....(X)(X-1) = (5)(4) = 20

GMAT assassins aren't born, they're made,
Rich

ScottTargetTestPrep · Answer

We can simplify the left side of the equation and then factor 162 as 3^4 * 2^1. Then we have:

3^x - 3^x * 3^-1 = 3^4 * 2^1

3^x(1 - 3^-1) = 3^4 * 2^1

On the left side, note that the expression (1 - 3^-1) = 1 - (⅓) = ⅔. We now have:

3^x(2/3) = 3^4 * 2^1

3^x = (3^4 * 2)(3/2)

3^x = 3^4 *3

3^x = 3^5

x = 5

So x(x-1) = 5(4) = 20.

Alternate Solution:

Note that 3^x = 3 * 3^(x - 1). Then:

3^x - 3^(x - 1) = 3 * 3^(x - 1) - 3^(x - 1)

Let’s factor the common 3^(x - 1):

3 * 3^(x - 1) - 3^(x - 1) = 3^(x - 1)(3 - 1) = 3^(x - 1)(2) = 162

3^(x - 1) = 81 = 3^4

x - 1 = 4

x = 5

Then, x(x - 1) = 20.

Answer: C

BrentGMATPrepNow · Answer

Given: 3^x - 3^(x-1) = 162
Factor to get:    = 162
Simplify to get:    = 162 
Divide both sides by 2 to get: 3^(x-1) = 81
Rewrite the right side as 3^(x-1) = 3^4
So, x - 1 = 4
This means x = 5

We get x(x - 1) = (5)(5 - 1) = (5)(4) = 20

Answer:  C

RELATED VIDEO FROM OUR COURSE
 https://www.youtube.com/watch?v=LHz74_lvLI8

EMPOWERgmatRichC · Answer

Hi All,

We're told that 3^(X) - 3^(X-1) = 162. We're asked for the value of then (X)(X-1). This question can be solved rather easily with a bit of 'brute force' arithmetic.

Since we're dealing with 'powers of 3', let's map out the first several values: 
3^1 = 3 
3^2 = 9 
3^3 = 27 
3^4 = 81 
3^5 = 243

We're subtracting two consecutive powers of 3 and ending up with 162. Looking at the list so far, we have an obvious 'pair' of values that fits what we're looking for:

3^5 and 3^4 
243 - 81 = 162

Thus, X = 5 and the answer to the question is (5)(5-1) = 20

Final Answer:  C

GMAT assassins aren't born, they're made, 
Rich

fskilnik · Answer

? = x\left( {x - 1} ight)

\left. \begin{gathered}
 {3^x} - {3^{x - 1}} = {3^{x - 1}}\left( {3 - 1} ight)\,\,\, \hfill \
 162 = 2 \cdot 81 = 2 \cdot {3^4} \hfill \ 
\end{gathered} ight\}\,\,\,\,\,\,\,\mathop \Rightarrow \limits^{{	ext{stem}}} \,\,\,\,\,\,\,{3^{x - 1}} = {3^4}\,\,\,\,\,\,\,\mathop \Rightarrow \limits^{{	ext{3}}\,\, 
otin \,\,\left\{ {0,1, - 1} ight\}} \,\,\,\,\,\,\,x - 1 = 4\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,? = 20

This solution follows the notations and rationale taught in the GMATH method.

Regards, 
Fabio.

jfranciscocuencag · Answer

Hello ScottTargetTestPrep !

Would you be so kind and explain to me how did you get to the following?

Note that 3^x = 3 * 3^(x - 1). Then:

3^x - 3^(x - 1) = 3 * 3^(x - 1) - 3^(x - 1)

Kind regards!

ScottTargetTestPrep · Answer

So here are some more details:

3^x = 3^  = 3^  =  *  = 3*3^(x - 1)

However, that's a long and complicated way of putting things. In reality, you would just observe that given any power, you can always decrease the exponent by one and multiply with the base to get an equivalent expression. For instance, if you have 2^5 (which is 2 multiplied by itself five times), that is equal to  *2 (which is 2 multiplied by itself four times, and then multiplied by 2 once more).

In the next line, all we did was to replace 3^x by the equivalent expression of 3*3^(x - 1) which we obtained as explained above.

Posted from my mobile device

bumpbot · Answer

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.

If 3^x - 3^(x-1) = 162, then x(x - 1) =

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

If 3^x - 3^(x-1) = 162, then x(x - 1) =

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards