def marginal_tax(income, rate, lower, upper):
    '''Calculate the marginal tax on income within a specific bracket.
    Income is the total amount of income, lower and upper are the
    boundaries of the tax bracket, rate is the tax rate within that bracket.
    Signature: (int, float, int, int) --> float
    '''
    if income <= lower:
        bracket_income = 0
    elif upper == None or income < upper:
        bracket_income = income - lower
    else:
        # income > upper
        bracket_income = upper - lower
    return bracket_income * rate
    

def effective_tax(income):
    '''Return effective tax rate given the following marginal rates:
    0% up to 10k
    10% up to 30k
    25% up to 60k
    35% up to 100k
    50% over 100k
    Signature: (int) --> float'''
    taxes = 0
    taxes += marginal_tax(income, 0.1, 10000, 30000)
    taxes += marginal_tax(income, 0.25, 30000, 60000)
    taxes += marginal_tax(income, 0.35, 60000, 100000)
    taxes += marginal_tax(income, 0.5, 100000, None)
    return taxes / income * 100
    

def fruit_purchase(is_ripe, on_sale):
    '''Return the number of pieces of fruit to purchase,
    given whether the fruit is ripe and/or on sale.
    Ripe and on sale --> buy 10
    Ripe, not on sale --> buy 5
    Not ripe --> buy 0
    Signature: (bool, bool) --> int'''
    if is_ripe and on_sale:
        return 10
    elif is_ripe:
        return 5
    else:
        return 0
    
    # nested version
    if is_ripe:
        # yes, the fruit is ripe
        if on_sale:
            return 10
        else:
            return 5
    else:
        # not ripe
        return 0