How do I calculate GST inclusive?

When adding 10% to the price is relatively easy (just multiply the amount by 1.1), reverse GST calculations are quite tricky:

1. To figure out how much GST was included in the price you have to divide the price by 11 ($220/11=$20);
2. To work out the price without GST you have to divide the amount by 1.1 ($220/1.1=$200)