Chaps & Saddles, a retailer of tack and Western apparel, earns an average contribution margin of 30 percent on its sales volume. Recently, the advertising manager of a local country radio station offered to run numerous radio advertisements for Chaps & Saddles at a monthly cost of $2,100. Compute the amount by which the proposed radio advertising campaign must increase Chaps & Saddles’s monthly sales volume to

a. Pay for itself.
b. Increase operating income by $1,500 per month.