A science fiction writer is calculating the number of possible unique 4-digit identification codes for Mars colonists, where each code must be divisible by 5. How many such positive 4-digit numbers are there?

Sat, 02 May 2026 16:10:29 +0000

**How Many Unique 4-Digit Identification Codes Could Mars Colonists Have? A Surprising Math in Space Exploration** As Mars missions edge closer to crewed arrival, fascination with identity systems for colonists has turned into more than science fiction. A quiet but growing curiosity stirs: if each Mars colonist needs a secure 4-digit identification code for access and registration, how many unique options exist? What math lies behind such a system—and why does it matter? The calculation centers on a simple but powerful boundary: the set of 4-digit numbers divisible by 5. In the U.S. and globally, 4-digit IDs typically range from 0000 to 9999—though in realistic terms, colonist codes may operate within tighter limits. By strict numerical rule, 4-digit whole numbers start at 1000 and end at 9999. From here, the logic becomes clear: a number is divisible by 5 if its last digit is either 0 or 5. This means only two possible endings for full compliance.