The question that started this thread: “Blue Book, page 919 prob. 16. If xy=7 and x-y=5, then x^2y-xy^2 = ??? a. 2, b. 12, c. 24, d 35, e. 70…”
shows up ALL THE TIME. Do enough SAT prep, and you’ll see it over and over again.
Basic rule for SAT prep: if it can be factored, factor it. If it can be multiplied out, multiply it out.
It’s like the problem of the number of poles needed to hold up a 300 foot wire if there’s a pole every 50 feet. You have to remember to include the last pole. And it shows up all the time!
Do enough SAT prep, and you’ll recognize the questions that show up on a regular basis.