I have to agree with griff. If you are going to puzzle someone you do expect to get the workings.
I posted it here as i was 100% sure my calculations were correct. I wanted to see who got it the same as me and Alfie. It is interesting that both here and there there were many a conflicting answer

The negative numbers thing pee'd me off a tad. I myself wanted the full explanation of the so say correct answer.