Feather Friends, Incorporated, distributes a high-quality wooden birdhouse that sells for $20 per unit. Variable expenses are $8 per unit, and fixed expenses total $180,000 per year. its operating results for last year were as follows: 6. The president does not want to change the selling price. Instead, he wants to increase the sales commission by $1 per unit. He thinks that this move, combined with some increase in advertising, would increase this year's unit sales by 25%. How much could the president increase this year's advertising expense and still earn the same $36,000 operating income as last year