def pva(a, i, n):
    """ This function is for the present value of money
    invested per period at the end of the month over a 
    period of time. 
    a = Periodic Investment (1000)
    i = interest rate as decimal (.0675)
    n = the number of compound periods (20)
    Example: pva(1000, .0675, 20)
    """
    return a / i * (1 - 1 / (1 + i) ** n)
