Updating Variables

Hello, I have a question that I do not understand why I have to answer. annual rain (from September) if I only ask for it in September and December, but if I put it, it puts me wrong, and I do not understand why I have to put it every month.
Could someone explain it to me?

september_rainfall = 5.16

annual_rainfall += september_rainfall

october_rainfall = 7.20

annual_rainfall += october_rainfall

november_rainfall = 5.06

annual_rainfall += november_rainfall

december_rainfall = 4.06

annual_rainfall += december_rainfall

https://www.codecademy.com/courses/learn-python/lessons/python-syntax/exercises/update-variables

that’s not the correct link. please provide the correct link so we can see the instructions, etc.

The exercise task is taken from Learn Python 2 - Python Syntax
url: https://www.codecademy.com/courses/learn-python/lessons/python-syntax/exercises/update-variables

Task

1.
We’re trying to figure out how much it rained in the past year! Update the annual_rainfall variable to include the values from September to December.

SOLUTION

january_to_june_rainfall = 1.93 + 0.71 + 3.53 + 3.41 + 3.69 + 4.50
annual_rainfall = january_to_june_rainfall

july_rainfall = 1.05
annual_rainfall += july_rainfall

august_rainfall = 4.91
annual_rainfall += august_rainfall

september_rainfall = 5.16
october_rainfall = 7.20
november_rainfall = 5.06
december_rainfall = 4.06

# Add the remaining months:
# September, October, November and December
annual_rainfall += september_rainfall
annual_rainfall += october_rainfall
annual_rainfall += november_rainfall
annual_rainfall += december_rainfall

Basically you have to add the remaining months: September, October, November and December. We are using augmented assignment

annual_rainfall += december_rainfall

as a shorthand for

annual_rainfall = annual_rainfall + december_rainfall