We want to use Mr. Anderson's balance to create a situation where opposite quantities combine to make 0.
First, for a given number A, its opposite is defined as:
(-1)*A = -A
Here we know that Mr. Anderson's balance is -$80.23
The opposite of his balance is:
-(-$80.23) = $80.23
So, if he decides to add $80.23 to his account, now the balance of the account will be:
original balance + what he adds
-$80.23 + $80.23 = $0
So he is adding the opposite of what he debts, in order to cancel the debt.
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