1. Is there a quick way to work out how much an item costs after GST at 15% has been added on?

Yes, multiply by 9 and divide by 8. (In other words multiply by (100 + 15)/100 which simplifies to 23/20).

2. When multiplying mixed numbers should they be changed to improper fractions?

Yes, it is almost always easier to do this first and then to cancel.

3. When working out percentage profit and loss problems is the ORIGINAL amount always used?

Yes. This applies to working out discount percentages too.