Hi there,

I have a question about the notation used in the transportation section of the "taking a vacation" lesson.

When using if/elif to add discount to car rental cost depending on the days, i would have assumed the code would look like this:

```
def rental_car_cost(days):
carcost = days * 40
if days >= 7:
carcost - 50
elif days >= 3:
carcost - 20
return carcost
```

However, this doesn't work. But when you copy the notation from the example and change the if/elif outcomes from "carcost - number" to "carcost -= number" it works:

```
def rental_car_cost(days):
carcost = days * 40
if days >= 7:
carcost -= 50
elif days >= 3:
carcost -= 20
return carcost
```

I would just like to know why my first code wouldn't work, and what " -= " or "+=" means.

Thanks for your help!