I am stuck on this exercise.

My code is:

```
def hotel_cost(days):
return 140 * days
def plane_ride_cost(city):
if city == "Charlotte":
return 183
elif city == "Tampa":
return 220
elif city == "Pittsburgh":
return 222
elif city == "Los Angeles":
return 475
def rental_car_cost(days):
cost = 40 * days
if days >= 7:
cost = cost - 50
elif days >= 3:
cost = cost - 20
return cost
def trip_cost(days,city):
return rental_car_cost(days) + plane_ride_cost(city) + hotel_cost(days)
```

The error I get is:

```
Oops, try again.
rental_car_cost(1) returned None instead of the correct value 40
```

I don't know how to correct it.