7 x 13 = 28

Byunggyu's result equals $A$ multiplied by the sum of the decimal digits of $B$. We can find this sum by extracting the digits one at a time.

Start with $X=B$ and $S=0$. While $X$ is positive, repeat the following operations:

• Add the remainder of $X$ divided by $10$ to $S$.
• Replace $X$ with the integer quotient of $X$ divided by $10$.

When the loop ends, print $A \times S$. Let $d$ be the number of digits of $B$. The time complexity for one case is $O(d)$, and the space complexity is $O(1)$.

Solution written by GPT6