645 makes 80 new (+ 5 used ones left over)
those 80 new will then be used making 80 (+ 5) = 85 used ones.
85 used ones make 10 new (+ 5 used left over)
tose 10 new will become used making 10 (+ 5) = 15 used
15 used ones will make 1 new one (+ 7 left over)
That one will become used and added to the 7 used ones already left over makes 8 - enough to make one more cup.