P = i^n * P - d * (i^n -1 / i -1)
n 0
can be written as:
def morgage(PStart, interest, monthlyPayment, numPayments):
"""Compute money remaining after a given number of downpayments given the start value and interest."""
return interest**numPayments * PStart - montlyPayments *((intrest**numPayments - 1) / (intrest - 1))