Macro to automatically add rows and copy formulas

[Dave]

I have a loan amrtization file that I use to evaluate loans. I have it set
up to allow me to change the number of periods and the payment automatically
recalculates and flows through to the amortization schedule. The problem is
if you go from 20 periods to 30 periods you have to copy the formulas in the
amortization schedule down 10 more rows to calculate the rest of the periods.

Is there a macro that would automatically add rows and copy the formulas for
the schedule down the necessary number of rows? I have an input section and
cell A5 is where I input the number of periods for the loan. Currently, the
20th period of a loan is on Row 28 and the columns used in the loan
calculation go from A-G.

Is there any way to automate the file to automatically update?

Thanks
Dave