Descartes' Cove is listed for grades 6 and up but DS has been working on the easiest level (Measurement) and he can understand the problems conceptually. In terms of mechanics, they use decimals and percents but part of my son's excitement about it is discovering these new things which he views as some kind of secret knowledge, lol. When he gets to something new, we look at the secret book for clues. DD played it a bit and was able to do the Measurement problems without too much help from me. She doesn't seem to be as interested in it as DS, though.
We haven't tried the harder levels, I don't really know what's on them. You can look at a demo of the game on the CTY website. It's a six part thing including Measurement, Number & Operations, Data Analysis & Probability, Algebra, Geometry, Reasoning & Proof.
