點解八達通一定要入$50,$100,$150,.....$1000(10分)

點解八達通一定要入$50,$100,$150,.....$1000『點解一定要係整數呢?????
點解唔可以增值$10,$20,$25,$30,$40..........$999。
點解最少要入五十蚊呢???????????
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