Step-by-step explanation:
Wally takes out $40 dollars out of his bank account every week, then we know that he has $100 dollars to begin with,
Meaning that for 4 weeks, we have to $40 dollars out of $100.
:
$100 - $40
==> $60
:
$60 - $40
==> $20
:
$20 - $40
==> -$60
:
-$60 - $40
==> -$100