Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:
st1 is not sufficient:
consider x=-5 y=5 ; x=5 y=5
in both cases st1 holds but stem is either "yes" or "no"...
so this is the ultimate proof of insufficiency of st1.
it is a bit difficult to expand st1 algebraicly and people here did various mistakes....
x^2-y^2 = (x+5)(y-5) = xy + 5y - 5x -25 ..... leads to nothing.
it is of utmost importance in gmat to know when to stop seeking algebraic expansions/solutions (and start look at other directions for the answer).
in this case the (something +5)*(something -5) is put as diversion that leads you to make mistakes - either doing wrong algebraic operations or assuming wrong things....
in fact this was a "red alert" for me. the stem doesn't say anything about specific numbers, just x and y. and then comes statement 1 and adds numbers to the equation... this sounded ood to me and made me extra alerted.