C++ Transaction fee should apply to every transaction separately

C++ I have attached to this post a a google documnt with the assignment I had to do with C++ I finished the assignment and I got a garde of 90 on it because "transaction fees should apply to every transaction separetly interest calculated is always 0 " Can somebody help me fix that ?

https://docs.google.com/document/d/1YRz3DC37881NZ1OjSnVy2tzeRbRofq6qCeT81uXhJVE/edit?usp=sharing