Brooke works part-time as a waitress in a restaurant. For groups of seven or more customers, the customer is charged 15% of the bill for Brooke's services. For parties of less than seven, the tips are voluntary. Brooke received $11,000 from the groups of seven or more and $7,000 in voluntary tips from all other customers. Using the customary 15% rate, her voluntary tips would have been only $6,000. Brooke must include $18,000 ($11,000 + $7,000) in gross income.

a. true
b. false