Data for Hermann Corporation are shown below:

Per Unit Percent of Sales

Selling price $ $75100\%$

Variable expenses $5168$

Contribution margin $ $2432\%$

Fixed expenses are $$75,000$ per month and the company is selling $4,000$ units per month.

Required:

$1-$a$.$ How much will net operating income increase $($decrease$)$ per month if the monthly advertising budget increases by $$9,900,$ the monthly sales volume increases by $100$ units, and the total monthly sales increase by $$7,500?$

$1-$b$.$ Should the advertising budget be increased?