I am trying to make the following functionality:

Two cells:
1. cell A1 which in it I will write a price without VAT.
2. cell A2 which in it I will write a price with VAT.

The functionality will be:
1. If I am writing the price without VAT in A1, the price will automatically
be calculated into A2 and will be written with VAT.
2. on the contrary of step 1.

How am I doing this ?

Another problem I encountered is:
When I write in A1 the price, I am overriding the formula of the cell.
Do I have any option to define a formula for a cell that enables me to write
a number in the cell without overriding the formula that is defined to that
cell ?

Thanks, Ido