Every turn is the same block, so a part-full window is a countdown: divide the free space by what one turn costs and you know how many are left. Marking the blocks in is the slow way to the same answer, and worth doing once, because it shows what the division is counting. The block is not the same for every agent, though: one that answers briefly gets far more turns out of the same window than one that writes at length. Notice too that the last turn has to fit whole; half a turn is not a turn.
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