Anyone know how to use excel to create tax spreadsheets but on mac numbers?

Hi All,
Finally a non health anxiety question and so close to XMAS, so not sure who will be on.

I am just wondering does anyone know the formula for calculation GST @ 15% and TAX @ 20% on cells?
I’m wanting the whole of one cell to be responsible for tax and another to be responsible for GST. So Cell A is amount – B is Tax and C is GST.
I can’t figure out how to do it!
Anyone know?