OK, thanks, I get he wants to "Just to pay his mortgage"
Where
m = Mortgage
b = Bills
d = Disposable income (have left to live)
t = Total income available
Now acceptable bills are a subjective concept really, but for simplicity we will assume them to be a fixed unavoidable amount.
Given
d = t -(m+b)
For the purposes of this question, the opening poster has proposed a solution for the value of
d=0. If we substitute that in:
0 =
t-(m+B)
Therefore
t=m+b
So there isn't anything left afterwards. The minimum value of
d when d=0 is 0