GMAT Progress Diary - From 680 to 7??

Extreme Challenge Set - Jeff Sackmann

So after completing Jeff Sackmann's Total GMAT Math, I moved on to Extreme Challenge Set.
If you have read reviews by other users you will know by now that the Extreme Challenge Set is very heavily oriented towards Probability & combinations.
I believe, Total GMAT Math and the EC set compliment each other very well. TGM focuses on each topic individually and then the EC set puts it all together to give you 700+ level questions.

So the EC set is split into 2 sections -
1) Problem Solving - questions 1 to 65
2) Data Sufficiency - questions 66 to 100.

I attempted to solve all the questions in under 2 mins. Before I started working on the questions, I split the PS and DS sections into 2 parts each:
PS: (1-35) & (36-65)
DS: (66-80) & (81-100)

I first solved PS (1-35). About 7 of them were based on Combinations + some other section. About 13 of them were based on Probability + some other section. A few geometry question and a sprinkle of number properties questions. Obviously I cannot classify each question into 1 or 2 exact categories. In the 700+ level questions, categories melt into each other.

Questions: 35
Wrong: 12 (4 careless, 7 tough, 1 Knowledge gap)
Accuracy: 66%

Then i picked up DS (66-80). Few inequalities, few NP's, a few coordinate geometry based questions and of course probability and combinations.

Questions: 15
Wrong: 6 (3 careless, 3 tough)
Accuracy: 60%

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After reviewing all the questions (right and wrong), I went back to TGM, pulled up the Probability & Combination questions and tried solving them. They seemed so much more simpler now. Hah.
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Yesterday night I picked up PS (36-65). Many of the questions are based on the concepts learnt in the first half.

Questions: 30
Wrong: 8 (4 careless, 3 tough, 1 not sure)
Accuracy: 73%

Today morning I picked up the remaining DS (81-100) questions.

Questions: 20
Wrong: 4 (1 careless, 2 tough, 1 not sure)
Accuracy: 80%

Overall result:
So overall, I solved 100 questions (all timed) and got 30 questions wrong (15 - tough, 12 - careless, 2 - not sure, 1 knowledge gap).
Final result: 70%.

I am not too happy. I wanted to score in the 80- 85% range. And I could have easily done that by cutting down on my careless errors. 3 to 4 of them as stupid as me marking the wrong answer after solving it exactly right!! I deserve a special title just for this! What I am happy about is that my accuracy increased in the 2nd half as a result of what I learned in the 1st half. I am also much more confident solving Probability and combinations questions. Working through this set has boosted my overall confidence and I think it was definitely worth $25.