credit card bisection

def payYear(balance, minpayment, monthlyinterest, upper, lower, epsilon):
    while abs(balance) <0.01:
        month = 1
        while month <13:
            balance = (balance - minpayment) * ( 1 + monthlyinterest)
            month = month + 1
        if balance > 0.01:
            upper = minpayment
        else:
            lower = minpayment
        minpayment = (upper+lower)/2.0
    return minpayment

balance = 320000
annualInterestRate = 0.2
epsilon = 0.01
monthlyinterest = annualInterestRate/12
lower = balance/12
upper = (balance*(1+monthlyinterest)**12)/12
minpayment = (upper+lower)/2.0
ans = payYear(balance, minpayment, monthlyinterest, upper, lower, epsilon)
print "minpayment is: " + str(round(ans, 2))