Credit card bisection 2

def payYear(startbalance, balance, minpayment, monthlyinterest, upper, lower):
    while balance > 0.01:
        month = 1
        while month <13:
            balance = (balance - minpayment) * ( 1 + monthlyinterest)
            month +=1
        if balance > 0.01:
            balance = startbalance
            upper = minpayment
        elif balance < 0:
            lower = minpayment
        minpayment = (upper+lower)/2.0
    return minpayment

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