$[$The following information applies to the questions displayed below.$]$ Data for Hermann Corporation are shown below: Per UnitPercent of SalesSelling price$ $90100\%$Variable expenses$6370$Contribution margin$ $2730\%$ Fixed expenses are $$78,000$ per month and the company is selling $3,500$ units per month$\mathrm{.}2-$a$.$ Refer to the original data. How much will net operating income increase $($decrease$)$ per month if the company uses higher$-$quality components that increase the variable expense by $$4$ per unit and increase unit sales by $20\%\mathrm{.}2-$b$.$ Should the higher$-$quality components be used?