This may be a set question or it may be the description of a problem you have chosen yourself.
Use appropriate techniques such as plain English, pseudo-code or structure diagrams, dry runs, code blocks (procedures and functions) to describe the solution. Imagine you are explaining the algorithm to a group of people who know less than you.
Include analysis of error trapping, eg when user enters inappropriate data.
Include full code.
Devise a strategy, don't just throw data at the window. What data is appropriate to test the program? Use ordinary data, exceptional data and false data (eg letters instead of numbers).
Limitations of the current solutions. Comment on ways you might extend or improve the program in future.