What is the point of adding -= instead of just -


#1

For example,
if days >= 7:
total - 50
Gives an error
But,
if days >= 7:
total -= 50
Is the right answer
Since they both subtract 50 from the total


#2

when you write

total - 50

it does not save(update) the value anywhere hence you cannot get an hold of the current value of total after subtracting 50 an use it
but
when you write

total -= 50 # same as writing total = total- 50

Here the value of total is updated(saved) to whatever it becomes after you take away 50 and you can use the new value

Hope this helps