The best black box test design technique for verifying the accuracy of this user story is equivalence partitioning. Equivalence partitioning is a technique that divides the input domain of a system into classes or groups that are expected to behave similarly. By testing one value from each class, the tester can reduce the number of test cases while still achieving good coverage. In this case, the input domainof the system is the amount of income, which can be divided into classes based on the tax percentage applied to different income ranges. For example, if the tax percentage is 10% for income below 10,000, 20% for income between 10,000 and 20,000, and 30% for income above 20,000, then the equivalence classes are: low income (<10,000), medium income (10,000-20,000), and high income (>20,000). By testing one value from each class, such as 5,000, 15,000, and 25,000, the tester can verify that the system calculates the correct tax percentage for each income range. This technique is more efficient and effective than testing all possible values of income, or testing only one value of income, or testing the states of income entry, or testing the statements in income calculation. References: ISTQB Foundation Level Agile Tester Syllabus1, Section 2.3.1, page 19; ISTQB Foundation Level Agile Tester Extension Sample Exam Questions2, Question 5, page 6.