def marginal_tax(income, lower, upper, rate):
    '''calculate the marginal tax paid within a given tax bracket'''
    if income <= lower:
        return 0
    elif upper == None:
        return (income - lower) * rate
    elif income > upper:
        return (upper - lower) * rate
    else:
        return (income - lower) * rate


def effective_tax(income):
    '''return effective tax rate given the following marginal tax rates:
    0% up to $10k
    10% up to $30k
    25% up to $60k
    35% up to $100k
    50% over $100k'''
    
    taxes = 0
    taxes += marginal_tax(income, 0, 10000, 0.0)
    taxes += marginal_tax(income, 10000, 30000, 0.1)
    taxes += marginal_tax(income, 30000, 60000, 0.25)
    taxes += marginal_tax(income, 60000, 100000, 0.35)
    taxes += marginal_tax(income, 100000, None, 0.5)
    return taxes / income
    

def fruit_purchase(is_ripe, on_sale):
    '''return the number of pieces of fruit you will purchase,
    given whether the fruit is ripe and/or on sale'''
    if is_ripe:
        if on_sale:
            return 10
        else:
            return 5
    else:
        # not ripe
        return 0


def last_element(some_list):
    '''return the last element of some_list'''
    if len(some_list) > 0:
        return some_list[len(some_list) - 1]
    else: # handle case of empty list
        return None