Given:x\(y^{3}\) = 24a + 12
This implies that:x\(y^{3}\) = 12(2a) + 12 = 12 (2a+1) = 12k
i.e., x\(y^{3}\) is a multiple of 12.
Observe what this means for y:x\(y^{3}\) contains \(2^{2}\) x 3, but not \(2^{3}\) x 3.
Let's think about this - what does the above tell us about x and y?
Can y contain a 2? No. If y contained even one "2", x\(y^{3}\) would contain an 8. Given that this number definitely contains a 3 as well (because it is divisible by 12), the number would then become divisible by 24 (thanks to the 8 and the 3). Inference: y contains no "2". Hence,
y is an odd number.
Observe what this means for x:If y is an odd number but x\(y^{3}\) is divisible by 12, then, definitely, x has to be even. Not only that, x has to contain the \(2^{2}\) that is necessary to get 12. Also, again, x cannot contain three or more 2's, because then, the number would become divisible by 24, given the presence of 3.Inference: x is even. Also, x contains
exactly two 2's.
So, what we have learned so far ->
- y is an odd number
- x is an even number, a multiple of 4, but not a multiple of 8 (i.e., it contains \(2^{2}\), but not \(2^{3}\))
Statement 1: x + y is oddx is even. y is odd. Hence, x + y = even + odd = odd. This is definitely true.
Reject choice B and choice C.Statement 2: x/2 is evenx = \(2^{2}\) (something). When divided by 2, a 2 remains in the numerator, making x/2 definitely even. So, this statement is also true.
Reject choice A.
Statement 3: x + y is a multiple of 3.
- y only needs to be an odd number. It may or may not contain 3.
- x only needs to be a multiple of 4. It may or may not contain 3.
So,
If x is a multiple of 3 and y is not ->
x = 3a
y = 3a + 1 (for example - we can also take an example with 3a+2).
x + y = 3k + 1.
In simple words, x + y, in such a case, will not be a multiple of 3. So,
statement 3 is not a must.
Reject choice E.
The answer is choice D.
Sidenote: We could quickly take sample numbers meeting the conditions and quickly invalidate statement 3, if needed.
Case 1: y is a multiple of 3 but not x
Example -> x = 4; y = 3
x\(y^{3}\) = 108. When divided by 24, 108 gives a remainder of 12. x + y = 4 + 3 = 7 (not a multiple of 3).
This is an example of (3b+1) + (3a) = 3k+1, as discussed above.
Thus, statement 3 is not a must.
Case 2: x is a multiple of 3 but not y
Example -> x = 12; y = 5
x\(y^{3}\) = 1500. When divided by 24, 1500 gives a remainder of 12. x + y = 12 + 5 = 17 (not a multiple of 3).
This is an example of (3b+2) + (3a) = 3k+2, as discussed above.
Case 3: Both x and y are multiples of 3
Of course, in such a case, x+y will also be a multiple of 3.
Example -> x = 12; y = 3
x\(y^{3}\) = 324. When divided by 24, 324 gives a remainder of 12. x + y = 12 + 3 = 15 (multiple of 3).
This is an example of (3b) + (3a) = 3k.Hope this helps!
Harsha