无法计算tripcost，而是抱怨NoneType

Question

def hotelcost(nights):
    cost = 140
    return nights * cost

def planeridecost(city):
    if city == "Charlotte":
        return 183
    elif city == "Tampa":
        return 220
    elif city == "Pittsburgh":
        return 222
    elif city == "Los Angeles":
        return 475

def rentalcarcost(days):
    cost = 40 * days
    if days >= 7:
        cost - 50
    elif days >= 3:
        cost - 20
    else:
        return cost
def tripcost(city,days,spendingmoney):
    return planeridecost(city) + hotelcost(days) + rentalcarcost(days) + spendingmoney
    print tripcost
    return tripcost

tripcost("Los Angeles",5,600)

我不能用Python解决这个问题。我尝试了其他几种代码来执行相同的操作，但无法执行：

^{pr2}$

Answer 1

对于days>；=7和days>；=3的情况，Rentalcarcost函数返回None。按如下方式执行：

def rentalcarcost(days):
    cost = 40 * days
    if days >= 7:
        return (cost - 50)
    elif days >= 3:
        return (cost - 20)
    else:
        return cost

tripcost函数有两个返回值，第二个返回值无效。在