Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » GMAT Prep Math


 Page 1 of 1 [ 5 posts ] 



  Print view Print view Previous topic Previous topic | Next topic Next topic
 

If x and y are positive integers, what is the value of x + y

Author Message
rrampriya
 Post subject: If x and y are positive integers, what is the value of x + y
 Post Posted: Mon Nov 16, 2009 7:18 pm 
Offline
Course Students


Posts: 4
If x and y are positive integers, what is the value of x + y

1. (2^x)(3^y) = 72
2. (2^x)(2^y) = 32

statement 1 leads to 1 solution for x and y :x=3,y=2
however statement 2 leads to 2 solutions: x=3,y=2 and x=1,y=4, u can also add x=2,y=3 and x=4,y=1.

Hence I selected A as my choice.

However, answer is D. How? Can someone explain? Thanks!!
Top 
gurm43
 Post subject: Re: If x and y are positive integers, what is the value of x + y
 Post Posted: Mon Nov 16, 2009 11:16 pm 
Offline
Course Students


Posts: 1
The question asks to find x+Y not indiviual values of x and y, for all the possible values for x and y in statement 2 the sum (x +y) still equals 5.
Top 
ULHASV
 Post subject: Re: If x and y are positive integers, what is the value of x + y
 Post Posted: Tue Nov 17, 2009 1:23 pm 
Offline
Students


Posts: 3
rrampriya wrote:
If x and y are positive integers, what is the value of x + y

1. (2^x)(3^y) = 72
2. (2^x)(2^y) = 32

statement 1 leads to 1 solution for x and y :x=3,y=2
however statement 2 leads to 2 solutions: x=3,y=2 and x=1,y=4, u can also add x=2,y=3 and x=4,y=1.

Hence I selected A as my choice.

However, answer is D. How? Can someone explain? Thanks!!


Hi,

The thing is, no need to be so engraved in what is the individual value of x and y, when the question is asking about x+Y, then be it 3+2, or 4+1(as you suggested), still you are getting definite answer "5", and this is asked. Why to trouble unnecessarily when you already got the answer.
By the way , 2^(x+y) = 2^5, how come you got 4 and 1 thing?

Thanks.
Anurag.
Top 
rrampriya
 Post subject: Re: If x and y are positive integers, what is the value of x + y
 Post Posted: Tue Nov 17, 2009 7:36 pm 
Offline
Course Students


Posts: 4
Thanks all! It was my bad!! I didnt read the qs properly!! :(!!
Top 
RonPurewal
 Post subject: Re: If x and y are positive integers, what is the value of x + y
 Post Posted: Fri Dec 18, 2009 6:14 am 
Offline
ManhattanGMAT Staff


Posts: 7146
read the questions carefully, guys. be sure to double-check exactly what they want you to find.

by the way:

(1) is a PRIME FACTORIZATION of 72.
since 2 and 3 are primes, we know that this is THE prime factorization of 72.

once you make this realization, you can deduce at once that (1) will give unique values of x and y, since it's impossible for a number to have more than one prime factorization.

--

(2) can be combined using basic exponent rules (multiply 2 powers with the same base --> add the exponents) to give 2^(x+y) = 32. at this point you can see that it's possible to find (x + y), even if you don't know the exact value from memory.

so, (d)
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 5 posts ] 





Who is online

Users browsing this forum: No registered users and 0 guests

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to: