EXERCISE $5\mathrm{.}9.$ Lin Corporation has a single product whose selling price is $$120$ and variable expense $$80$ per unit. The company's monthly fixed expense is $$60,000.$

a$.$ Using the equation method, what are the unit sales required to earn a target profit of $$10,000?$

b$.$ Using the formula method, what are the unit sales required to earn a target profit of $$15,000?$