Number Prop frpm GMAT prep Test

[shrey2287]

If x and y are integers.Is x(y+1) even.

1. x and y are prime nos
2. y > 7

ans : C.. (Both Stmts are required)

Plz explain..

[GmatKiss]

If x and y are integers.Is x(y+1) even.

1. x and y are prime nos
2. y > 7

ans : C.. (Both Stmts are required)

Plz explain..

From, 1.
x and y can be any of (2,3,5,7,11,13,17,19...)

lets sub: x(y+1)

x=2; y=5
2(6) =12 - even
x=3; y=2
3(3) = 9 - odd

NOT SUFFICIENT

2. is clearly not sufficient, (we dont know what is X)

NOT SUFFICIENT

1+2 combined,

x can be (2,3,5,7,11,13,17,18....)
y can be (11,13,17,19...)

If you sub, you will know that x(y+1) is always even.

SUFFICIENT.

IMO:C

[tpr-becky]

This questin tests the concept of even/odd when two numbers are multiplied together if one of them is even then the product is even.

Therefore we need to know if one of x or (y+1) is even - thus the question is "is x even or is y odd?" if we know the answer to either of these then we can solve.

1) if they are both prime then they could be 2, 3, 5, ... - we don't know for sure whether x is even and we don't know for sure that y odd therefore this is insufficient.

2) y>7 - this tells us nothing about x and y can be even or odd so this is insufficient.

if we know both then we know that y is a prime greater than 7 - the only primes greater than 7 are odd numbers. Therefore we know for certain that y is odd and can determine that the product from the stem will be even. That means the correct answer is C.