Complete the analogy 15 : 8 :: 21 : ? by finding the rule that links the first pair.

Break the number into its two prime factors and add them: $15=3\times 5$ and 3 + 5 = 8. For $21=3\times 7$ the total is 3 + 7 = 10. The trap 21 comes from multiplying the two prime factors instead of adding them, which merely hands the number back. 4 is 7 − 3, subtracting where the rule adds. 3 is the digit total of 21, a rule that fails the first pair because 1 + 5 = 6, not 8.