GST (Goods and Services Tax) is charged at the rate of 15% in New Zealand.
15% is the same as 15/100 as a fraction or 0.15 as a decimal.
To calculate how much GST has to be added you need to multiply the cost by 0.15. This is then added to give the GST inclusive cost.
If you don’t have your phone handy can can you do the calculation with pen and paper?