Thanks to John! Sept 2015 Session

That's great - many congratulations :-)

I do not know what you mean by the 'indefinite interest rate'!! There is no such standard term, and it has never ever been relevant in any exam question. There is more than one way of setting out the answer to this sort of question, but the net result is the same. I set it out in the way I think is the most logical (and it would get full marks).