This calculation assumes that the 10 person-days are absorbed by a single assignee. Counting 10 working days from June 13 does give Jun 13, 16, 17, 18, 19, 20, 23, 24, 25, and 26, that is June 26; but the report layout is assigned to two people, Sato and Tanaka, so the duration required is 10 person-days / 2 people = 5 days. A finish date of June 26 therefore ignores the number of assignees and is wrong.
○イ
正解The correct answer is イ. First check the calendar: June 1 falls on a Sunday, and no work is done on Saturdays or Sundays, so the working days are Jun 2-6, Jun 9-13, Jun 16-20, Jun 23-27, and Jun 30. Now follow Tanaka's tasks in order. The screen list starts on June 3 and takes 4 person-days, so it uses the 4 working days Jun 3, 4, 5, and 6 and finishes on June 6. The screen flow that follows starts on June 9 and takes 6 person-days, so it uses the 6 working days Jun 9, 10, 11, 12, 13, and 16 and finishes on June 16. Meanwhile, the report layout is assigned to two people, Sato and Tanaka, with an effort of 10 person-days, and Mr. S set it to start on June 13 and finish on June 19. The working days in that period are Jun 13, 16, 17, 18, and 19, that is 5 days, and 5 days x 2 people = 10 person-days, so the effort arithmetic alone does work out. However, Tanaka is assigned to the screen flow up to June 16, so on June 13 and June 16 he would be doing the screen flow and the report layout at the same time, which is a double assignment. Because the work overlaps with the assignee's preceding task, this schedule cannot secure two people's worth of effort, and the finish date cannot be set to June 19. That is the appropriate criticism from the senior colleague.
×Incorrectウ
Focusing on the fact that two people are assigned is correct, but the number of days falls short. From June 13 to June 18 there are only 4 working days, Jun 13, 16, 17, and 18, and 4 days x 2 people = 8 person-days, which does not cover 10 person-days. Two people need 5 working days to complete 10 person-days, so the earliest possible finish is June 19. The claim that the finish date can be set to June 18 therefore does not hold.
×Incorrectエ
A work duration is determined by effort divided by the number of people assigned, so a task does not need a longer duration merely because its effort is larger than that of other tasks. In this table too, the 10 person-days are kept within 5 days by splitting them between two people. Moreover, this is only general advice and does not point out any concrete error contained in Mr. S's schedule, so it is not an appropriate criticism of an error.
ここがポイント
To compute a work duration, use effort (person-days) / number of people assigned = number of days required, then lay those days out on the calendar counting only working days, excluding Saturdays, Sundays, and holidays. Note that the start date itself counts as one day.
The validity of a schedule must be checked not only by the effort arithmetic but also by each person's availability. If the same assignee is already occupied with another task during the same period, the result is a double assignment (an overloaded resource), and a schedule that works on paper cannot actually be carried out.