The Math Club at school is 70% girls. The girls number 5 more than twice the number of boys in the club. How many boys and how many girls are in the club?

So, if x*y > than zero, we know that neither x nor y can be zero, because anything multiplied by zero is equal to zero.  Now, we also know, that a negative times a negative is positive, and a positive times a positive times a positive is positive.

So,

x and y can either both be negative, or both be positive.  

They cannot be zero, or one negative, one positive.

Hope this helps!