Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

What's wrong if i solve the problem as follows: 3*3x=3*9^y 3x = 9^y [Dividing both side by 3] x = 3^(2y-1) Ans. C. Please help to correct my wrong concept as the OA is E.
_________________

What's wrong if i solve the problem as follows: 3*3x=3*9^y 3x = 9^y [Dividing both side by 3] x = 3^(2y-1) Ans. C. Please help to correct my wrong concept as the OA is E.

The point is that 3*9^y does not equal to 27^y: 3*9^y=3*3^{2y}=3^{2y+1} on the other hand 27^y=(3^3)^y=3^{3y}.

Solution: If 9x = 27^y, which of the following expresses x in terms of y? A. 3^y B. 3^(y-1) C. 3^(2y-1) D. 3^(2y-3) E. 3^(3y-2)

9x=3^2*x and 27^y=(3^3)^y=3^{3y} --> 3^2*x=3^{3y} --> x=\frac{3^{3y}}{3^2} --> x=3^{3y-2}.

What's wrong if i solve the problem as follows: 3*3x=3*9^y 3x = 9^y [Dividing both side by 3] x = 3^(2y-1) Ans. C. Please help to correct my wrong concept as the OA is E.

Bunuel has already pointed out your error so I will not repeat it.

Let me give you another method of working out the solution here (though you should ensure that you understand the theory of exponents well)

If 9x = 27^y, which of the following expresses x in terms of y? A. 3^y B. 3^(y-1) C. 3^(2y-1) D. 3^(2y-3) E. 3^(3y-2)

We need x in terms of y. In 9x = (27)^y, put y = 0. You get x = 1/9

Now see which option will give you 1/9 when you put y = 0. I hope you can quickly see that only option (E) will give you x = 1/9 when y = 0. Answer is (E). Such methods work well when you have variables in the options.

Note: If more than one options had given 1/9, you could have tried some other values to choose the right answer out of those options.
_________________

Re: If 9x=27^y, which of the following expresses x in terms of y [#permalink]
08 Apr 2012, 21:41

To Karishma,

I'm glad you used the same approach as I've used, but I used y=1, which makes, 9x = 27^y 9x = 27 x = 3. Look for options which give ans. as x=3, I marked A (option E also gives x=3), which gives x=3 which is wrong ans. Can u explain where I'm wrong

Re: If 9x=27^y, which of the following expresses x in terms of y [#permalink]
09 Apr 2012, 04:28

Expert's post

priyalr wrote:

To Karishma,

I'm glad you used the same approach as I've used, but I used y=1, which makes, 9x = 27^y 9x = 27 x = 3. Look for options which give ans. as x=3, I marked A (option E also gives x=3), which gives x=3 which is wrong ans. Can u explain where I'm wrong

Thnx, Priyal

When you put y = 1, three of the five options give you x = 3 (options A, C and E). Any one of these 3 could be the correct answer. You now need to try out some other values of y to get the answer out of these 3 options.

You need to check all the options to ensure that no other options gives you the same value. When I put y = 0, I try it in all the options and only option E gives me 1/9 so I can directly mark that as the answer. This is the reason I mentioned the note in the post above: "Note: If more than one options had given 1/9, you could have tried some other values to choose the right answer out of those options."

So now your next step is to put y = 0 and out of the 3 options, only E will satisfy x = 1/9. Try and put the easiest value first. Easiest value is generally 0, if allowed.

Putting y = 2/3/4... will make it cumbersome.
_________________