I moved into an apartment complex where the key is both unique to each apartment door, but every key can open the front door, how are both possible?

Question

I moved into an apartment complex where the key is both unique to each apartment door, but every key can open the front door, how are both possible?

In: 986

Answer 1

Think of it like a 5 number combination lock. All the apartment keys can open combination 1-2-3–x-x. The front door only requires the first three digits. Your key could be 1-2-3-7-5. So every key starts with 123 allowing them to open the front door.