4. (1.5 point) Martha Stanley sells three items (which it purchases from a supplier): bath towels, hand towels, and washcloths in a 4:3:2 mix (thus, a batch of 9 towels has 4 bath towels, 3 hand towels, and 2 washcloths). Each bath towel sells for $10 and costs $4, each hand towel sells for $5 and costs $2; and each washcloth sells for $2.50 and costs $1. The shop's annual fixed expenses are $324,000. a. What is the weighted-average contribution margin per unit? b. How many bath towels must the firm sell at the breakeven point