Great series! Also the hardest for me by far, and my review is similar to sspenst's Cool puzzle to work out how to do. Undo makes it easy if you make an error and realize once you 'get there' Thought I was missing something on a completely wrong path, and then also caught the 2 off in the correct path first I knew what I was meant to do, but still found it tricky to see! Tricky. 66 a ton of ways, then got to 64, but realized a quick way to cut 2 in that route 2 99's before the 97, which felt slower. Nice! Wow, didn't think that would work 
Insights