hello there! i came to this section to ask the same question about the same delta course
i actually learned about it from other gmat forum, where a person who got 760 posted her review this spring:
"...in the last few weeks before my exam I signed up for the material at
https://www.deltacourse.com. It's online only, and you can't easily print it - you have to go through it one page at a time. But the price is pretty reasonable ($27), and it certainly helped me get a better grasp of this stuff. You can also sign up for their free GMAT problem of the day (sent by email).
Their website is a bit heavy on the hype - for example, their "advanced algebra" stuff turns out to be one single article about sqrt(-1). But overall I think their material was worth it - at least for me (I'm 46, and it's been a looong time since I last saw a factorial)"
... "I think comb/perm is freaking a lot of people out unnecessarily. I only had one combinations question where it really helped to know the xCy formula (the number of ways to choose y elements from a set of x elements). On a second comb/perm question, the trick was more to translate the word problem into a comb/perm concept - the actual answer was small enough that I could have just enumerated the possible outcomes. For probabilities, just the basics were sufficient - nothing beyond what's covered in the math review that's in the beginning of
OG. My feeling is that you are far more likely to need solid arithmetic and algebra skills (including exponents) than esoteric comb/perm/probabilities. "
so i really don't know if the course is worth buying... if anybody could share the experience with the course, i'd appreciate that...