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Bunuel
The expression \(2a^2 +2b^2\) can be written in the form of \(18x + y\), where a, b, x and y are non-negative integers and \(y < 18\). Is \(|y + 3| = 3\)?

(1) The difference between a and b can be expressed as an even multiple of 3.
(2) b when divided by 3 is an integer.


Are You Up For the Challenge: 700 Level Questions


So, we have some info in the question stem, and we should first work on it.

|y+3|=3 gives two values of y..
1) y+3=3....y=0,
2) y+3=-3.....y=-6...NOT possible

SO the question becomes - ' Is y=0?'

Also it is given that \(2a^2+2b^2=18x+y\)

(1) The difference between a and b can be expressed as an even multiple of 3.
So, a-b=3p, where p is an integer..
The link between a-b and a^2+b^2 should tell us that we are looking at squaring a-b..
\((a-b)^2=9p^2\)....
\(a^2+b^2-2ab=9p^2\)....
\(2a^2+2b^2-4ab=18p^2\)....
\(18x+y-4ab=18p^2\)....
\(y-4ab=18(p^2-x)=18s\)
We can only say that y will be EVEN, and y-4ab is a multiple of 18.

(2) b when divided by 3 is an integer.
b=3q

Combined.
since \(a-b=3p\) and \(b=3q\), a will also be a multiple of 3, say \(a=3r\)
\(y-4ab=18s\)....
\(y-4*3q*3r=18s\)....
\(y-36qr=18s \)....
\(y=18s+36qr=18(s+2qr)=18t\).
This means y is a multiple of 18 for sure. But \(0\leq{y}<18\), so only possibility is y=0 as 0 is a multiple of 18.
Our answer is YES.
Suff

C
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Solving Info available:-

2(a^2+b^2) = 18x +y > Even = Even + Even
Implies y can only be Even no.

|y+3| = 3 > y=0, y=-6,
Only Positive value allowed, only possible solution> y=0

Hence a^2 +b^2 = Should be multiple of 9

Option 1) :-

a-b = 6N
Squaring both sides

a^2+b^2 -2ab = 36N^2
>a^2 +b^2 = 36N^2 +2ab implies 9A+ (2ab~ can be multiple of 9 in case when a or b are multiple of 3) ------(1)
* A is some positive integer

Not Sufficient

Option 2 :-

b=3M

a^2 +b^2 = 9M^2 +a^2, only possible when a is multiple of 3 * M is some positive integer

Not Sufficient

Combining both

a-b=6N
b=3M

> a= 3(2N+M)

a^2 +b^2 = will be off form 9G + constant
* G is some positive integer

Hence Sufficient to proof as multiple of 9
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