Consider a credit card number consisting of 6 digits (ranging from 1 to 9). The first two digits are 15 in that sequence, the third digit must be greater than 5, the fourth digit needs to be a multiple of 3, and the fifth digit is twice the sixth digit. How many unique credit card numbers can be formed?

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Official Explanation

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A credit card number consists of 6 digits (from 1 to 9). The first two digits are fixed as 15. The third digit can be 6, 7, 8, or 9, giving us 4 options. The fourth digit, being a multiple of 3, can be 3, 6, or 9, providing 3 choices. The fifth digit is twice the sixth…

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Comments

SriMay 4, 2025

good question

VasistApr 16, 2024

thanks!

SandraJan 21, 2025

Good question

What this question tests

What it tests

Whether you can count arrangements and selections correctly — permutations vs. combinations.

Common trap

Overcounting: treating a selection as ordered when order does not matter, or forgetting to divide by duplicates.