I had one that I enjoyed many years ago, to write a service that calculated the Nth digit of Pi.

It was up to you what you wanted to do / how / if you wanted to deploy. They only asked that you write how you would've liked to receive such service from another org.

Then in the day of the in person interview we reviewed my code / README / tests ; was very nice.

Could you use a lookup table?

Did you have the option to represent Pi in a hexidecimal base?

That speeds things up no end.