Yes. it is very clever..
it works becasue you are always left with a certain pattern of numbers from your subtraction calculator... i.e. 108 or 801 or 297 or 927 etc, etc etc... so say for example your left with 297 or 927 or 792, by entering 27, it knows that the answer is 9... the only exception to this rule is 0 as 0 appears in a couple of patterns, that is why they tell you not to circle 0(not because it already is a circle...
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