If the "80" here is reasonably accurate, then here is what we have: in 80% of the cases, we only have to focus on 80% of stuff (that's about 64% of the total),

If the "80" here is reasonably accurate, then here is what we have: in 80% of the cases, we only have to focus on 80% of stuff (that's about 64% of the total), and in the rest 20% we focus almost on 100% of stuff (that's almost 20% of the total), which means that we need to focus on almost 64+20=84% of the total, which is still pretty close to the original 80%. :-) OK, this is silly. (We can only get it equal when we go with 100% instead of 80%.)