I’ve a question regarding the usage of ( -= ) here. Why should we place an ( = ) sign after the ( - ) sign? Shouldn’t just placing a ( - ) be enough?

The question was to subtract 50 from the cost if the days were 7 or greater, and 20 if it was 3 or greater.

def rental_car_cost(days):

cost = 40 * days

```
if days >= 7:
cost -= 50
elif days >= 3:
cost -= 20
return cost
```