Numeric system with negative digits are interesting in this respect. If you read 11.67 as actually 12.3̅3̅ (with 3̅ = -3), then it becomes clear that you need

Numeric system with negative digits are interesting in this respect. If you read 11.67 as actually 12.3̅3̅ (with 3̅ = -3), then it becomes clear that you need to give a ten and two bills, and get some change. Or said otherwise, when paying, you should round up in your mind, not down...