Cullumber Corporation sells a single product for $$40.$ Its management estimates the following revenues and costs for the year $2022$:

Net sales

$$420,000$

Selling expenses$-$variable

$$16,800$

Direct materials

$75,600$

Selling expenses$-$fixed

$16,800$

Direct labour

$50,400$

Administrative expenses$-$variable

$8,400$

Manufacturing overhead$-$variable

$16,800$

Administrative expenses$-$fxed

$8,400$

Manufacturing overhead$-$fixed

$25,200$

$($a$)$

Assume the price remains at $$40$ per unit and variable costs remain the same per unit, but fixed costs increase by $16\%$ annually.

Calculate the percentage increase in unit sales required to achieve the same level of annual operating income calculated in part second part. $($Round answer to $2$ decimal places, e$.$g$.15\mathrm{.}25\%.)$

Percent increase in unit sales$\%$