Sorry

Sorry, I'm a complete beginner... even in programming. I understand the exemple and I can figure out that 115 should be the right number to withdraw because 120 - 115 - 0.5 -1% = 3.35 = 3 but I don't understand what I have to do in the code part ? Do I have to test numbers ? create a algorythm which doing it for me ? should i write something like... withdraw < 120, withdraw/5 = integer then do balance - (balance - 5) and if the result is 0