The mistake is setting the 3-dimension total to 80 cm. 80 falls in the “up to 80” row, so it corresponds neither to column b (3-dimension total = up to 100) nor to column c (also up to 100). In fact, b's (A, 80, 10) is the test data for column 2 and c's (A, 80, 20) is the test data for column 3 (the column of a), so both are shifted one size-category block (three columns) to the left. By the convention of Table 2, the boundary value 80 is read as belonging to the “up to 80” side, not to “up to 100”.
○イ
正解The correct answer is イ. First, pin down which columns of Table 2 b and c are. From the arrows in the figure, b is column 5 and c is column 6, and the Yes marks are: for b, “District = A / 3-dimension total = up to 100 / Weight = up to 10”, and for c, “District = A / 3-dimension total = up to 100 / Weight = up to 20”. The important point here is that the 3-dimension-total rows and the weight rows in Table 2 are mutually exclusive ranges. A parcel of 80 cm or less falls in the “up to 80” row, so what corresponds to the “up to 100” row is 81 to 100 cm. Likewise, for weight, “up to 5” means 1 to 5 kg, “up to 10” means 6 to 10 kg, and “up to 20” means 11 to 20 kg. Therefore the test data for b must be District A, 3-dimension total 81 to 100 cm, weight 6 to 10 kg, and c must be District A, 3-dimension total 81 to 100 cm, weight 11 to 20 kg. In イ, b (A, 90, 10) has 90 cm within 81 to 100 and 10 kg within 6 to 10, so it matches column 5 exactly. c (A, 90, 11) has 90 cm within 81 to 100 and 11 kg within 11 to 20, so it matches column 6. As a further check on the expected outputs: for b, the 3-dimension total gives category 2 and the weight gives category 2, so category 2 applies and District A costs 1,600 yen; for c, the 3-dimension total gives category 2 and the weight gives category 3, so the larger category 3 applies, giving 2,000 yen. This confirms that the two adjacent columns properly form tests that verify different categories.
×Incorrectウ
The weight of 11 kg for b does not fall under “up to 10” but under “up to 20”, so this is data for column c, not for column b. Moreover, c's 21 kg violates requirement (1), which says that a parcel weighing more than 20 kg is not accepted, so it does not exist in any column of Table 2 in the first place and is data that cannot be accepted. It is a double error: b is shifted one column to the right, and c falls outside the table altogether.
×Incorrectエ
It is correct that a 3-dimension total of 100 cm falls under “up to 100”, but the weight correspondences are all shifted one row earlier. b's 5 kg falls in the “up to 5” row, so it is column 4; c's 10 kg falls in the “up to 10” row, so it is data for column 5 (the column of b). Even if you match only the 3-dimension-total condition, if the weight range does not match the specified row, it does not become a test of that column.
ここがポイント
Read the condition rows of a decision table (output result table) as mutually exclusive: “up to 100” is not 0 to 100 but 81 to 100, the values that do not fall into the row above it, “up to 80”. Likewise, “up to 10” is 6 to 10 kg and “up to 20” is 11 to 20 kg. When mapping test data to columns, the biggest point is not to get this boundary split wrong: which row you place a boundary value such as 80, 5, or 10 into shifts the column by one.
For the size category, apply whichever is larger: the category obtained from the 3-dimension total or the category obtained from the weight. Also, a parcel whose 3-dimension total exceeds 140 cm or whose weight exceeds 20 kg is not accepted, so data such as 21 kg does not correspond to any column of Table 2. Table 2 is an expected-value table covering 4 districts × 3 three-dimension-total ranges × 3 weight ranges = 36 combinations, and it is a typical preparation step for black-box testing, which verifies by the correspondence between inputs and expected outputs without looking at the internal structure.