Write 4*5*p in a shorter form

that'll be 20p as 4 x 5 is 20

The correct equation to use is as follows:

where A is the amount after t years, P is the principal, r is the annual interest rate expressed as a decimal and n is the number of times the interest is compounded in a year.
Plugging in the given values we get:

After 15 years the investment will be worth $13,253.90