I'm going to assume "couldn't accomplish the task" means couldn't get it entirely correct. Do you provide a testing baseline that determine the expected behaviour? With TDD being the norm and correct way to test your program behaviour, I'm absolutely not going to go through every edge case with a fine tooth comb to check your "correctly formatted number" calls the API as you expect for a homework assignment. Provide a testing baseline or clearly demarcate the various possible edge cases as would be discussed in real life.