I’m not certain of exactly what you’re asking here (and I don’t do Excel on a Mac in any case), but it seems that you want to have a cell – that is, a single cell – to calculate and add the GST and Tax amounts on a value to be entered into another cell.
Without knowing anything about the rules that apply to this tax, or whether it’s an additive tax (that is, if the value of the second tax is incremented by the inclusion in the taxable value of the first tax), here are some things that I would do:
1. Let’s say for purposes of this example that the value on which we’re going to calculate the tax is at cell A2.
2. I would suggest that you include separate cells to clearly show the tax rates for the GST and Tax amount percentages, at 15% and 20% respectively. (Those can be at B2 and C2, with labels at B1 and C1 to show what they are.) That’s because someday those tax rates may change, and it’s always best in your spreadsheet design to have variable amounts in cells where the changes can be visible, rather than hard-coded into the formulas, where they often get lost or overlooked when the world changes.
3. Assuming the tax amounts are NOT additive, and that each tax is computed only on the original value, and assuming that we want a single cell for “GST and Tax amount” as you have stated, I’ll suggest putting that formula at A3 for now. The formula at A3 could be:
=A2 * ( B2 + C2)
Obviously, that’s the Windows formula – the only formula writing I know in Excel. If the Mac formula composition is different, I’m sure you can figure the difference and enter such a simple formula.
That formula will give you a value… according to the numbers that you have provided… of the value in A2 * 0.35, or 35% of your starting value.
If the tax will be additive, and the second tax is figured on the original value + the amount of the first tax, then the formula will be different, but you’ll have to say so.