How to design any base multiplication?

Question

To design a base multiplication
That's
Sample input
Base is 5
Digit 1 :-1220
Digit 2 :- 31
Output
43320

Answer 1

It depends on what you're allowed to do. As sugested by Ipang you could convert numbers from a base to decimal numbers, calculate product of them and convert the result back into the base.
Here an example with base 2:

https://code.sololearn.com/cbq12cROX96D/?ref=app

Answer 2

1220 in base 5 is 185 in decimal
31 in base 5 is 16 in decimal
How come result was 43.320?

Did I calculate it wrong or misunderstood?