Your first line does not make sense. Are you multiplying Gross_amt by .05 to get the net_amt? If so, then
net_amt := Gross_amt * .05;
but the second line would read something like this in a formula
New_Gross_amount := Net_amt/.05
Of course it is an assumption the New_Gross_amount and net_amount would be variables and thus needs to be defined (i.e., numbervar New_Gross_amount; );