I take GMAT Prep test and get stuck with this question. Please help!
If p and n are positive integers and p>n, what is the remainder when p2 -n2 is divisible by 5?
1) The remainder when p+n is divisible by 5 is 1
2) The remainder when p-n is divisible by 3 is 1
Thank you for your help!!
Integers - experts please help
This topic has expert replies
GMAT/MBA Expert
- Brent@GMATPrepNow
- GMAT Instructor
- Posts: 16207
- Joined: Mon Dec 08, 2008 6:26 pm
- Location: Vancouver, BC
- Thanked: 5254 times
- Followed by:1268 members
- GMAT Score:770
NOTE: You have transcribed the original GMATPrep question incorrectly in a few places. The corrections are made in red.VyDinh wrote:If p and n are positive integers and p>n, what is the remainder when p² - n² is divided by 15?
1) The remainder when p+n is divided by 5 is 1
2) The remainder when p-n is divided by 3 is 1
Target question: What is the remainder when p² - n² is divided by 15
Rephrased target question: What is the remainder when (p + n)(p - n) is divided by 15?
For this question, we're going to use a nice rule that says:
If N divided by D, leaves remainder R, then the possible values of N are R, R+D, R+2D, R+3D,. . . etc.
For example, if k divided by 5 leaves a remainder of 1, then the possible values of k are: 1, 1+5, 1+(2)(5), 1+(3)(5), 1+(4)(5), . . . etc.
Okay, onto the question . . .
Statement 1: The remainder when p+n is divided by 5 is 1
Since there's no information about p-n, statement 1 is NOT SUFFICIENT.
Statement 2: The remainder when p-n is divided by 3 is 1
Since there's no information about p+n, statement 2 is NOT SUFFICIENT.
Statements 1 and 2 combined
Statement 1: Applying the above rule, some possible values of p+n are 6, 11, 16, 21, 26, etc.
Aside: you'll notice that I didn't include 1 as a possible value since we're told that p and n are positive integers, and we can't get a sum of 1 if both are positive
Statement 2: Applying the above rule, some possible values of p-n are 1, 4, 7, 10, 13, etc
Let's examine two cases with conflicting results.
case a: p+n = 11 and p-n = 1
Add the equations to get 2p = 12, which means p = 6 and n = 5 (perfect, we have positive integer values for p and n)
In this case, when (p + n)(p - n) is divided by 15, the remainder is 11
case b: p+n = 6 and p-n = 4
Add the equations to get 2p = 10, which means p = 5 and n = 1 (perfect, we have positive integer values for p and n)
In this case, when (p + n)(p - n) is divided by 15, the remainder is 9
Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT
Answer = E
Cheers,
Brent
- 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
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
- vinay1983
- Legendary Member
- Posts: 643
- Joined: Wed Aug 14, 2013 4:27 am
- Thanked: 48 times
- Followed by:7 members
I think the option is E. given condition is p > n and we are asked (p^2-n^2)/5 = ? some value(one specific value)
Statement 1
the numbers satisfying this condition are 6,11,16,21,26,31
let's take 6 can be written as 5+1 only(condition p > n) then 5^2-1^2= 25-1=24/5=4 remainder
Let's take 11, which can be written as 6+5, then 6^2-5^2=36-25=11/5=1
not sufficient
Statement 2
the numbers can be 4,7,10,13,16,19,22,25,28,31
Let's take 4, i.e 6-2 or 5-1,
for 6-2 it is 6^2-2^2=36-4=32/5=2
for 5-1 it is 5^2-1^2=25-1=24/5=4, hence not sufficient
Combining both statements number 31 when divided by both 5 and 3 gices remainder 1
but 31 can be written as 26+5, 27+4, 20+11, 16+15
So let's take 26+5= 26^2-5^2= 676-25=651/5=1
Let's take 27+4= 27^2-4^2=729-16=713/5=3
no one value obtained hence insufficient
Statement 1
the numbers satisfying this condition are 6,11,16,21,26,31
let's take 6 can be written as 5+1 only(condition p > n) then 5^2-1^2= 25-1=24/5=4 remainder
Let's take 11, which can be written as 6+5, then 6^2-5^2=36-25=11/5=1
not sufficient
Statement 2
the numbers can be 4,7,10,13,16,19,22,25,28,31
Let's take 4, i.e 6-2 or 5-1,
for 6-2 it is 6^2-2^2=36-4=32/5=2
for 5-1 it is 5^2-1^2=25-1=24/5=4, hence not sufficient
Combining both statements number 31 when divided by both 5 and 3 gices remainder 1
but 31 can be written as 26+5, 27+4, 20+11, 16+15
So let's take 26+5= 26^2-5^2= 676-25=651/5=1
Let's take 27+4= 27^2-4^2=729-16=713/5=3
no one value obtained hence insufficient
You can, for example never foretell what any one man will do, but you can say with precision what an average number will be up to!
- vinay1983
- Legendary Member
- Posts: 643
- Joined: Wed Aug 14, 2013 4:27 am
- Thanked: 48 times
- Followed by:7 members
I think the option is E. given condition is p > n and we are asked (p^2-n^2)/5 = ? some value(one specific value)
Statement 1
the numbers satisfying this condition are 6,11,16,21,26,31
let's take 6 can be written as 5+1 only(condition p > n) then 5^2-1^2= 25-1=24/5=4 remainder
Let's take 11, which can be written as 6+5, then 6^2-5^2=36-25=11/5=1
not sufficient
Statement 2
the numbers can be 4,7,10,13,16,19,22,25,28,31
Let's take 4, i.e 6-2 or 5-1,
for 6-2 it is 6^2-2^2=36-4=32/5=2
for 5-1 it is 5^2-1^2=25-1=24/5=4, hence not sufficient
Combining both statements number 31 when divided by both 5 and 3 gices remainder 1
but 31 can be written as 26+5, 27+4, 20+11, 16+15
So let's take 26+5= 26^2-5^2= 676-25=651/5=1
Let's take 27+4= 27^2-4^2=729-16=713/5=3
no one value obtained hence insufficient
Statement 1
the numbers satisfying this condition are 6,11,16,21,26,31
let's take 6 can be written as 5+1 only(condition p > n) then 5^2-1^2= 25-1=24/5=4 remainder
Let's take 11, which can be written as 6+5, then 6^2-5^2=36-25=11/5=1
not sufficient
Statement 2
the numbers can be 4,7,10,13,16,19,22,25,28,31
Let's take 4, i.e 6-2 or 5-1,
for 6-2 it is 6^2-2^2=36-4=32/5=2
for 5-1 it is 5^2-1^2=25-1=24/5=4, hence not sufficient
Combining both statements number 31 when divided by both 5 and 3 gices remainder 1
but 31 can be written as 26+5, 27+4, 20+11, 16+15
So let's take 26+5= 26^2-5^2= 676-25=651/5=1
Let's take 27+4= 27^2-4^2=729-16=713/5=3
no one value obtained hence insufficient
You can, for example never foretell what any one man will do, but you can say with precision what an average number will be up to!