The correct answer is: $10$
Explanation
From the example Input 17 → Output 10 we deduce the rule is: subtract the product of the digits from the number.
Steps:
- Product of digits of 17: $1\times7=7$
- Subtract from 17: $17-7=10$
- Apply same rule to 13: product $1\times3=3$, then $13-3=10$
Therefore the output is $10$.