Interest is $1,320. P is the principal amount, $6,600.
r is the interest rate, 4% per year, or in decimal form, 4/100=0.04.
t is the time involved, 5 years time periods.
So, t is 5 year time periods.
I= p x r x t
In order to get the final number just add $1,320 + $6,600 which is $7,920