BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

5 x + 3 y = 17

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
GMAT Instructor
Posts: 3650
Joined: Wed Jan 21, 2009 4:27 am
Location: India
Thanked: 267 times
Followed by:80 members
GMAT Score:760

5 x + 3 y = 17

by sanju09 » Fri Aug 27, 2010 5:14 am
If 5 x + 3 y = 17, what is the value of x?

(1) x is a positive integer.

(2) y = 4 x.


[spoiler]Source: https://gmat.practicetests.teachstreet.com[/spoiler]
The mind is everything. What you think you become. -Lord Buddha



Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com
Top
Source: — Data Sufficiency |
User avatar
Legendary Member
Posts: 1309
Joined: Wed Mar 17, 2010 11:41 pm
Thanked: 33 times
Followed by:5 members

by pradeepkaushal9518 » Fri Aug 27, 2010 7:33 am
imo B

2 variable requires 2 equations
A SMALL TOWN GUY
Top
User avatar
Legendary Member
Posts: 659
Joined: Mon Dec 14, 2009 8:12 am
Thanked: 32 times
Followed by:3 members

by Gurpinder » Fri Aug 27, 2010 8:39 am
pradeepkaushal9518 wrote:imo B

2 variable requires 2 equations
Hey,

In this case, (B) is sufficient!

However, the rule 2 variables & 2 equations is NOT written in stone. It depends on case-per-case basis. Sometimes you can have 2 equations and still not be able to solve them. So just make sure you confirm that you are able to solve rather than simply look for 2 vars and 2 eq's.


Thanks!
"Do not confuse motion and progress. A rocking horse keeps moving but does not make any progress."
- Alfred A. Montapert, Philosopher.
Top
Senior | Next Rank: 100 Posts
Posts: 61
Joined: Fri Feb 19, 2010 10:12 am
Thanked: 3 times

by neerajbansal » Fri Aug 27, 2010 4:11 pm
D in my opinion..The meat of the question is to evaluate the choice A.

When the LCM of the x coeff and the y coeff added with the lowest of two is Greater than the constant, that equation has only one value..

5x+3y=17

lcm(5,3) = 15 add 3 to it...===> 18

18>17

so there is only one solution possible.
Top
User avatar
Legendary Member
Posts: 1309
Joined: Wed Mar 17, 2010 11:41 pm
Thanked: 33 times
Followed by:5 members

by pradeepkaushal9518 » Fri Aug 27, 2010 7:18 pm
5x+3y=17

x=1 y=4
x=2 y=7/3 ( no where given that y is integer)

do u think 1 is sufficient
A SMALL TOWN GUY
Top
GMAT Instructor
Posts: 1302
Joined: Mon Oct 19, 2009 2:13 pm
Location: Toronto
Thanked: 539 times
Followed by:164 members
GMAT Score:800

by Testluv » Sat Aug 28, 2010 1:50 am
However, the rule 2 variables & 2 equations is NOT written in stone. It depends on case-per-case basis. Sometimes you can have 2 equations and still not be able to solve them.
We can't use the n variables and n equations tactic when:

--equations aren't linear
--equations aren't distinct (the so-called "evil-twin" situation)
--we have integers-->this restricts the kinds of values variables can take such that you may have sufficiency even if you have fewer equations than variables

In general, the risk in using the tactic is concluding INSUFFICIENCY. So be careful before you conclude a statement is insufficient when using this tactic. But when concluding SUFFICIENCY, there isn't as much risk.

--------

The question stem provides us with 1 equation and 2 unknowns. Knowing nothing about the properties of the unknowns, we would need an additional equation.

But (1) tells us x is a positive integer. But, as Pradeep showed, y can be a non-integer. It can also be a negative number:

5(4) + 3(-1) = 17

Thus, (1) is insufficient. Eliminate A and D.

(2) provides us with an additional equation. Thus, we have two equations, and two unknowns, and, without solving, we can safely and quickly conclude that (2) is sufficient by itself.

Choose B.
Kaplan Teacher in Toronto
Top
User avatar
Legendary Member
Posts: 1309
Joined: Wed Mar 17, 2010 11:41 pm
Thanked: 33 times
Followed by:5 members

by pradeepkaushal9518 » Sat Aug 28, 2010 5:11 am
thanks testluv for clearing the confusion.
A SMALL TOWN GUY
Top
User avatar
Legendary Member
Posts: 659
Joined: Mon Dec 14, 2009 8:12 am
Thanked: 32 times
Followed by:3 members

by Gurpinder » Sat Aug 28, 2010 6:41 am
Testluv wrote: We can't use the n variables and n equations tactic when:

--equations aren't linear
--equations aren't distinct (the so-called "evil-twin" situation)
--we have integers-->this restricts the kinds of values variables can take such that you may have sufficiency even if you have fewer equations than variables
Hey,

Could you please clarify the third rule. I get the first 2 but unsure where you are getting at with the third one.

Thank you.
"Do not confuse motion and progress. A rocking horse keeps moving but does not make any progress."
- Alfred A. Montapert, Philosopher.
Top
Post new topic Post Reply
• Page 1 of 1