gmatprep上面的一道ds

hy903

if x and y are positive integers, is the product xy even?

1) 5x-4y is even

2) 6x+7y is even.

答案d

这题怎么做啊？

ProteinMonica

if xy even, then at least x or y is even

1) 5x-4y is even => 5x and 4y both are even (since 4y can not be odd,so 5x have to be even) = > at least x is even

2) same process as 1)

hy903

呀，原来这么简单啊！谢谢mm

zinra

嘿嘿～謝謝誒