※ Our check while writing this explanation could not fully confirm the published answer; scoring follows IPA's official key.
×Incorrectア
The shortening of 2 days is correct, but the additional cost is wrong. The total cost after the split is 5+6+4=15, and the difference from the 12 before the split is 3, not 2. Taking the additional cost as 2 comes from misreading the post-split total as 14 (for example, reading H2's 6 as 5), so add up 5, 6, and 4 to get 15 and compare it with 12.
×Incorrectイ
Both the additional cost of 2 and the shortening of 3 days are wrong. The cost difference is 3 and the shortening is 2 days. Taking the shortening as 3 days comes from miscalculating the post-split duration as 4 days, that is, overlooking the 3 days of H1 that runs in parallel and adding only H2's 2 days and H3's 2 days. The actual duration after the split is max(3,2)+2=5 days.
○ウ
正解Before the split, task H was 7 days with a cost of 12 (Table 1 of question group A). Looking at Table 2 for after the split, the costs are H1=5, H2=6, and H3=4, totaling 15, so the additional cost is 15-12=3. For the days, the key is the preceding-task relationship: H1 and H2 both have the same preceding tasks, C, E, and G, so once C, E, and G are finished, these two can start at the same time and proceed in parallel. Therefore both H1 and H2 are complete 3 days later, the duration of the longer one, H1. After that, H3, whose preceding tasks are H1 and H2, takes 2 days, so the duration after the split is 3+2=5 days. Compared with the 7 days before the split, this is a saving of 7-5=2 days, so ウ, which says that by adding a cost of 3 task H can be shortened by 2 days, is correct. The key point of the calculation is that tasks which can run in parallel are not added up; you take the longer one.
×Incorrectエ
The additional cost of 3 is correct, but the number of days shortened is wrong. A shortening of 3 days would require the duration after the split to be 4 days, but even though H1 and H2 can run in parallel, H3 must wait for both of them to finish, so it takes at least 3+2=5 days. The shortening stops at 2 days.
ここがポイント
Basics of the arrow diagram (PERT): tasks that share the same preceding tasks can run in parallel, and a following task waits until all of them are finished. The duration of the parallel part is not the sum but the maximum (here, max(H1=3, H2=2)+H3=2 = 5 days).
Evaluating the cost-versus-duration trade-off of splitting a task: the difference between the total cost after the split and the cost before it is the additional cost (15-12=3), and the difference between the durations before and after the split is the time saved (7-5=2 days).