If p, x, and y are positive integers, y is odd, and p = x^2

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If p, x, and y are positive integers, y is odd, and p = x^2 [#permalink]

14 Aug 2009, 12:49

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If p, x, and y are positive integers, y is odd, and p = x^2 + y^2, is x divisible by 4?

(1) When p is divided by 8, the remainder is 5.
(2) x – y = 3

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Re: PS: Divisible by 4 [#permalink]

14 Aug 2009, 13:46

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netcaesar wrote:

If p, x, and y are positive integers, y is odd, and p = x^2 + y^2, is x divisible by 4?

(1) When p is divided by 8, the remainder is 5.
(2) x – y = 3

SOL:

St1:
Here we will have to use a peculiar property of number 8. The square of any odd number when divided by 8 will always yield a remainder of 1!!

This means that y^2 MOD 8 = 1 for all y
=> p MOD 8 = (x^2 + 1) MOD 8 = 5
=> x^2 MOD 8 = 4

Now if x is divisible by 4 then x^2 MOD 8 will be zero. And also x cannot be an odd number as in that case x^2 MOD 8 would become 1. Hence we conclude that x is an even number but also a non-multiple of 4.
=> SUFFICIENT

St2:
x - y = 3
Since y can be any odd number, x could also be either a multiple or a non-multiple of 4.
=> NOT SUFFICIENT

ANS: A
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Re: PS: Divisible by 4 [#permalink]

15 Aug 2009, 13:49

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Very good solution

I did not know this property of 8. Kudos to you.

By and induction.
1^2=1 mod 8
say
n^2=1 mod 8 (n is an odd number)
than
if (n+2)^2=1 mod 8 ? (n+2 is the next odd number)
(n+2)^2=n^2+4n+4= 1 + 4n + 4 mod 8
4n+4=0 mod 8 because n is an odd number and 4n=4 mod 8.
So induction works.

So for any odd number n, n^2=1 mod 8

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Re: PS: Divisible by 4 [#permalink]

16 Dec 2010, 07:13

Can I ask someone to look at this question a provide a solution that doesn't depend on knowing peculiar properties of number 8 or induction?

Thank you.

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Re: PS: Divisible by 4 [#permalink]

16 Dec 2010, 07:39

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nonameee wrote:

Can I ask someone to look at this question a provide a solution that doesn't depend on knowing peculiar properties of number 8 or induction?

Thank you.

If p, x, and y are positive integers, y is odd, and p = x^2 + y^2, is x divisible by 4?

(1) When p is divided by 8, the remainder is 5 --> p=8q+5=x^2+y^2 --> as given that y=odd=2k+1 --> 8q+5=x^2+(2k+1)^2 --> x^2=8q+4-4k^2-4k=4(2q+1-k^2-k).

So, x^2=4(2q+1-k^2-k). Now, if k=odd then 2q+1-k^2-k=even+odd-odd-odd=odd and if k=even then 2q+1-k^2-k=even+odd-even-even=odd, so in any case 2q+1-k^2-k=odd --> x^2=4*odd --> in order x to be multiple of 4 x^2 must be multiple of 16 but as we see it's not, so x is not multiple of 4. Sufficient.

(2) x – y = 3 --> x-odd=3 --> x=even but not sufficient to say whether it's multiple of 4.

Answer: A.
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Re: PS: Divisible by 4 [#permalink]

18 Dec 2010, 11:23

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maliyeci wrote:

Very good solution

Its not something one shall already know before attacking a question, you may realize properties like this when u start solving a question.
Even I didn't know about this property of 8.

I approached the question in following way:

Stmt 1: P/8=(x^2+y^2)/8; using remainder theorem;
rem[(x^2+y^2)/8]= rem[x^2/8] + rem[y^2/8]
if x is divisible by 4, then x^2= 4k*4k= 16K=8*2K is also divisible by 8.
now to anaylze rem[y^2/8]; start putting suitable values of y; i.e all odd values starting from 1.
for y=1; rem(1/8)=1
for y=3; rem(9/8)=1
for y=5;rem(25/8)=1

so you observe this pattern here. :-D

coming back to ques now, as rem[(x^2+y^2)/8]= rem[x^2/8] + rem[y^2/8]=
rem[x^2/8] + 1 =5; this means rem[x^2/8] is not 0; which implies x is not divisible my 8;
Sufficient

Stmt2:
y being odd can be accept both 3 and 5 as values and we get different results; thus
Insufficient

Thus OA is A
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Re: PS: Divisible by 4 [#permalink]

19 Dec 2010, 07:49

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netcaesar wrote:

If p, x, and y are positive integers, y is odd, and p = x^2 + y^2, is x divisible by 4?

(1) When p is divided by 8, the remainder is 5.
(2) x – y = 3

Such questions can be easily solved keeping the concept of divisibility in mind. Divisibility is nothing but grouping. Lets say if we need to divide 10 by 2, out of 10 marbles, we make groups of 2 marbles each. We can make 5 such groups and nothing will be left over. So quotient is 5 and remainder is 0. Similarly if you divide 11 by 2, you make 5 groups of 2 marbles each and 1 marble is left over. So 5 is quotient and 1 is remainder. For more on these concepts, check out: http://gmatquant.blogspot.com/2010/11/divisibility-and-remainders-if-you.html

Coming to your question,

First thing that comes to mind is if y is odd, y^2 is also odd.
If y = 2k+1,
y^2 = (2k + 1)^2 = 4k^2 + 4k + 1 = 4k(k+1) + 1
Since one of k and (k+1) will definitely be even (out of any two consecutive integers, one is always even, the other is always odd), 4k(k+1) will be divisible by 8. So when y^2 is divided by 8, it will leave a 1.

Stmnt 1: When p is divided by 8, the remainder is 5.
When y^2 is divided by 8, remainder is 1. To get a remainder of 5, when x^2 is divided by 8, we should get a remainder of 4.
x^2 = 8a + 4 (i.e. we can make 'a' groups of 8 and 4 will be leftover)
x^2 = 4(2a+1) This implies x = 2*\sqrt{Odd Number}because (2a+1) is an odd number. Square root of an odd number will also be odd.
Therefore, we can say that x is not divisible by 4. Sufficient.

Stmnt 2: x - y = 3
Since y is odd, we can say that x will be even (Even - Odd = Odd). But whether x is divisible by 2 only or by 4 as well, we cannot say since here we have no constraints on p. Not sufficient.

Answer (A).
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Re: PS: Divisible by 4 [#permalink]

16 Jul 2012, 18:19

Am i missing something, why cant we take stmt 2 as follows:
squaring x-y=3 on both sides, we get p=9+2xy, that is p=odd + even = odd, not divisible by 4

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Re: PS: Divisible by 4 [#permalink]

16 Jul 2012, 23:24

Eshaninan wrote:

Am i missing something, why cant we take stmt 2 as follows:
squaring x-y=3 on both sides, we get p=9+2xy, that is p=odd + even = odd, not divisible by 4

The question is: "Is x divisible by 4?" not "Is p divisible by 4?"

x is even since y is odd. We don't know whether x is divisible by only 2 or 4 as well.
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Re: Number props - the answer provided was wrong, need clarif [#permalink]

29 Dec 2012, 03:59

arjun206 wrote:

If p, x, and y are positive integers, y is odd, and p = x2 + y2, is x divisible by 4?
(1) When p is divided by 8, the remainder is 5. (2) x – y = 3.

In my opinion the answer should be D, please provide explanations with your responses. Thanks!

Merging similar topics. Please refer to the solutions above.
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Re: If p, x, and y are positive integers, y is odd, and p = x^2 [#permalink]

03 Jan 2013, 22:59

good ques thanks!!

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Re: If p, x, and y are positive integers, y is odd, and p = x^2 [#permalink]

12 Jun 2013, 10:58

netcaesar wrote:

If p, x, and y are positive integers, y is odd, and p = x^2 + y^2, is x divisible by 4?

(1) When p is divided by 8, the remainder is 5.
(2) x – y = 3

1.
As p = 8I + 5
we have values of P = 5,13,21,29..... etc ..
as y is odd
when we solve this p(odd) = x^2 + y^2(odd)
x^2 = odd -odd = even

which can be 2,4,6 ... etc
but if we check for any value of p we don't get any multiple of 4.
so it say's clearly that x is not divisible by 4.

2.
x-y = 3
x = y(odd)+3
x is even which can be 2,4,6.. so it's not sufficient ..

Ans : A

Re: If p, x, and y are positive integers, y is odd, and p = x^2 [#permalink] 12 Jun 2013, 10:58

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If p, x, and y are positive integers, y is odd, and p = x^2

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