it's really easy to do cisco.... you could do it in your head even using a 12 digit number (although the person thinking of the number would have to use paper and pen to come up with the subtraction part of the problem....LOL)
all you do is add up all the digits after the smaller number has been subtracted, and they will always be a multiple of 9 ..... so when you see their answer
example 72534 ---- that totals 21 (so the number they circled has to be 6)
if their number already totals a multiple of 9 (the number they circled had to be either a 9 or a Zero, which is why they tell you not to circle 0's)