This question's explanation and your answers so far.
図は, リスクシミュレーションを基に, あるプロジェクトの見積りコストに対して最終的にその額に収まる確率を示したものである。現在, プロジェクトの予算として 4,000 万円を用意している。実際のコストが見積りコストを上回ってしまう確率を 20% まで引き下げるためには, 予備として, あとおよそ何万円用意することが妥当か。 （図: 横軸「見積りコスト (万円)」, 縦軸「見積りコストに収まる確率 (%)」の累積確率曲線。曲線上の主な対応点は 見積りコスト 4,000 万円で確率 15%, 4,800 万円で 50%, 5,000 万円で 80%, 6,000 万円で 90%。）
正解 The vertical axis of the figure is the probability of staying within the estimated cost, so the probability that the cost exceeds the estimate is 100% minus that probability. Lowering the probability of exceeding to 20% therefore means the same as raising the probability of staying within to 80%. Following the 80% horizontal guideline in the figure, the corresponding estimated cost is 5,000 man-yen. The current budget is 4,000 man-yen, so it is enough to add the shortfall of 5,000 − 4,000 = 1,000 man-yen as a reserve. The correct answer is ア, 1,000 man-yen.
Adding 2,000 man-yen makes the total 6,000 man-yen. In the figure, 6,000 man-yen corresponds to a 90% probability of staying within, that is, a 10% probability of exceeding, which is more coverage than the required 20%. The question asks approximately how many man-yen are appropriate, meaning the minimum reasonable amount, so there is no need to fund up to the 90% level.
4,800 man-yen is the estimated cost corresponding to the 50% point in the figure (a 50% probability of exceeding); it is not an additional amount. Even if read as an addition, the total would become 8,800 man-yen, far beyond the range of the graph. This is the typical mistake of taking a value on the horizontal axis directly as the answer.
An addition of 9,000 man-yen corresponds to a total of 13,000 man-yen, far beyond the region where the curve has already saturated near 100%. As a reserve it is clearly excessive, and the cost of the risk response is out of balance with its effect, so it is not appropriate.