GMATPrepNow
x and y are positive integers. When x is divided by 2y, the remainder is y. What is the value of y?
(1) x is even
(2) y is a prime number
*kudos for all correct solutions
Target question: What is the value of y? Given: x and y are positive integers. When x is divided by 2y, the remainder is y --------ASIDE------------------------------------
When it comes to remainders, we have 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.
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From the given information, we can say that the possible values of x are: y, y + 2y, y + 4y, 6 + 6y, . . . etc.
In other words,
the possible values of x are: y, 3y, 5y, 7y, . . . etc. Statement 1: x is even We already know that
the possible values of x are: y, 3y, 5y, 7y, . . . etc All of these possible x-values are in the form x = (ODD)(y)
So, if x is EVEN, it must be the case that
y is EVENThere are several values of x and y that satisfy statement 1. Here are two:
Case a: x = 12 and y = 4. In this case, the answer to the target question is
y = 4Case b: x = 6 and y = 2. In this case, the answer to the target question is
y = 2Since we cannot answer the
target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: y is a prime number We already know that
the possible values of x are: y, 3y, 5y, 7y, . . . etc So, there are several values of x and y that satisfy statement 2. Here are two:
Case a: x = 2 and y = 2. In this case, the answer to the target question is
y = 2Case b: x = 3 and y = 3. In this case, the answer to the target question is
y = 3Since we cannot answer the
target question with certainty, statement 2 is NOT SUFFICIENT
Statements 1 and 2 combined Statement 1 told us that
y is EVENStatement 2 told us that y is a prime number
Since 2 is the ONLY even prime number, the answer to the target question is
y = 2Since we can answer the
target question with certainty, the combined statements are SUFFICIENT
Answer: C
Cheers,
Brent