nmargot wrote:

Can someone show how this works? I used different numbers to the explanation given by

Kaplan and they dont work. It seems that you have to use numbers divisble by 4 for y where c - d = y, but how would I know this with out plugging a series of numbers.

nmargot : Here we have 4 variables. So it will be easy to use variable form instead of using numbers to find the correct answer.

For any question, try to check if variable option makes sense, if its tough,try by putting different values.

We have 2 methods to solve a question : variable or by using numbers. Always try to find which one will work better.

Here

If c + d = x and c – d = y, then cd

To find cd we have to write given equations in form of c and d

c+d =x

c-d =y

Adding both : 2c = x+y => c = (x+y)/2

Subtracting: 2d = x-y => d = (x-y)/2

Now multiple 2 equations to get value of cd => cd = (x+y)/2 * (x-y)/ 2 = (x^2-y^2)/4

Answer: D