Below your existing code, define a function called rental_car_cost with an argument called days.
Calculate the cost of renting the car:
Every day you rent the car costs $40.
if you rent the car for 7 or more days, you get $50 off your total.
Alternatively (elif), if you rent the car for 3 or more days, you get $20 off your total.
You cannot get both of the above discounts.
Return that cost.
def定义函数，然后if，elif，else定义各种情况，return返回结果