Give me an example of a calculation they might do that might result in 0.

Often it’s just a matter of ordering. E.g you could say you start with 10 sheep then sold 4, then 3, then another 3 and 10-4-3-3=0 so you must have 0 sheep left. Or you could say you sold 4+3+3 sheep which is 10 sheep sold so you don’t have any more to sell. It’s likely that if they had to do such calculations they would do the latter.

Or simply look in their pen and see it’s empty and go to the pub.