A terabyte is 1012 bytes. There are 9 digits in a SSN, hence 109 possible SSN's. Dividing gives 103 bytes per SSN.
If we compute a disk address directly from the SSN, we can access the record for any SSN with one (slow) disk access.
Although not all SSN's are valid, disk space is cheap, so it is okay to waste some of it.
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