Oodles of overlap. I don't remember the exact overlap from 3rd -> 4th, but from 5th -> 6th, there was 80%+ of the "pie" filled in to start.

Just for a sense of completion, we had our son push through to 100% full pie (ALEKS sent the "ready to move up" email after about 90%, I think). I didn't want him to save the least palatable for last and think he could get out of doing those.

Being offended is a natural consequence of leaving the house. - Fran Lebowitz