No, you've lost me there slowworm, what part of my comment is inaccurate then? Presuming it was my comment.
I understand the concept of compound interest. It just means interest gains interest doesn't it?
Ive just given that as an example above and said why think its wrong.
So what do you mean?
You reference to paying back 2 or 3 times what you borrow. This is simply down to compounding and would apply to whoever you borrowed from.
Your B&B example is also rather misleading, in the real world taxes would reduce the £50 to about £5 when it gets returned to the salesman. People would also expect to pay using cheques, credit cards etc further reducing the amount and proving banks do provide a purpose.