An email service has 500 million active users. Each receives 50 emails/day, and the average stored email (headers + body + small attachments amortized) is ~100 KB.
Use: 1 day = 86,400 sec ≈ 10^5 sec. Powers of 10: 1 KB = 10^3 B, 1 MB = 10^6 B, 1 GB = 10^9 B, 1 TB = 10^12 B, 1 PB = 10^15 B.
Compute daily volume and 1-year / 5-year storage in petabytes.