20 is the value you get if you refer to the wrong cell and calculate from the current total of 150 calls in D15 (150 ÷ 7.5 ＝ 20). The question asks for the number of operators after the measures are implemented, and the formula in column I is based on column H in the same row (the number of calls received after the measures are implemented), which is 158. In fact, in the row where H9 is 150 calls, I9 is exactly 20, so 20 is the answer for 150 calls, not the answer for 158 calls.
×Incorrectイ
21 is the value you get by truncating 158 ÷ 7.5 ＝ 21.07… downward (or by simply copying the 21 in G15, the current total number of operators). Staffing must never fall short, so rounding up is required: 21 persons can only handle 21 × 7.5 ＝ 157.5 calls, which is not enough for 158 calls. Even a shortfall of 0.07 of a person requires adding one more person, and that is the key point of this question.
○ウ
正解The correct answer is choice ウ, 22 persons.
Step 1: Find how many calls one operator can handle in one hour. From [Reception Situation] (3), one call takes an average of 8 minutes, so in one hour (60 minutes) an operator can handle 60 ÷ 8 ＝ 7.5 calls.
Step 2: a is cell I15, that is, the number of operators after the measures are implemented for the 20:00〜21:00 slot. In the same row, the number of calls received after the measures are implemented is the 158 calls in cell H15 (note that it is not the current total of 150 calls).
Step 3: The required number of people is 158 ÷ 7.5 ＝ 21.07… persons. The number of people must be a whole number, and 21 persons could only take 21 × 7.5 ＝ 157.5 calls, which cannot cover 158 calls. Therefore we round up, and 22 persons are needed.
Check: This formula of rounding up the number of calls divided by 7.5 matches every other row already displayed. For example, I4 is 82 ÷ 7.5 ＝ 10.93…→11, I5 is 91 ÷ 7.5 ＝ 12.13…→13, I9 is 150 ÷ 7.5 ＝ 20.0→20, I12 is 183 ÷ 7.5 ＝ 24.4→25, and I14 is 162 ÷ 7.5 ＝ 21.6→22, all exactly matching the values in the figure. Hence a ＝ 22.
×Incorrectエ
23 is an excessive value that overestimates the required staffing by one person. The capacity of 23 persons is 23 × 7.5 ＝ 172.5 calls, a large surplus compared with 158 calls. The rows that show 23 persons in the figure are those where column H is 166 calls (I10) or 168 calls (I13); 23 persons first become necessary only when the number of calls exceeds 22 × 7.5 ＝ 165. The 158 calls do not reach that level.
ここがポイント
Reading a formula in spreadsheet software: to find the value of a blank cell, identify the calculation rule that holds in common across the other rows of the same column and apply it. Here column I ＝ ROUNDUP(column H ÷ 7.5), and the key is not to mistake the source of the reference: it is column H in the same row (the number of calls received after the measures are implemented).
Headcount (capacity) calculation and rounding up: first convert units, 8 minutes per call → 60 ÷ 8 ＝ 7.5 calls per person per hour, then required headcount ＝ number of calls ÷ calls handled per person. For resources that cannot be divided, such as people, always round up when a fraction appears (rounding down would leave the capacity short).