Answer:
$36,000 increase
Explanation:
The computation of the  overall effect on the company's monthly net operating income of this change is shown below:
Particulars        Current          Proposed
Unit sales          9,000 units       9,400 units
Sales             $1,620,000        $1,692,000
                (9,000 units × $180)   (9,400 units × $180)
less: variable cost   -$324,000        -$470,000
                (9,000 units × $36)   (9,400 units × $50)
Contribution margin  $1,296,000       $1,222,000
Less: fixed cost      -$1,044,000      -$934,000
Net operating income  $252,000       $288,000
Hence, there is an increase in net operating income by
= $288,000 - $252,000
= $36,000