問80
頻出度★★★
稼働率0.9の装置を2台直列に接続したシステムに, 同じ装置をもう1台追加して3台直列のシステムにしたとき, システム全体の稼働率は2台直列のときを基準にすると, どのようになるか。
平成30年度 春期 ITパスポート試験 問80
テクノロジ系システム構成要素 › システムの評価指標
This question's explanation and your answers so far.
解説
問 80
Incorrectア
Adding one more device in series means multiplying by another availability value (0.9, which is less than 1), so the overall availability must go down. It cannot rise; the direction of change is the opposite.
Incorrectイ
With three devices in series, the two-device value is multiplied by another 0.9, so it definitely decreases from 0.81 to 0.729. The value does change, so “it stays the same” is wrong.
- ウ
正解 The availability of a series-connected system equals the product of the availabilities of its individual devices. Since a single device has an availability of 0.9, two devices in series give 0.9×0.9=0.81, and three devices in series give 0.9×0.9×0.9=0.729. Taking the two-device case (0.81) as the baseline, the three-device case is 0.729÷0.81=0.9 times that, i.e. it drops to 90% of the baseline. Therefore the availability “decreases by 10%”, so the correct answer is ウ.
Incorrectエ
Against the baseline of 0.81, the three-device value is 0.729, and 0.729÷0.81=0.9, so the decrease is 10%. A 30% decrease is a calculation error that overestimates the actual size of the drop.
ここがポイント
- The availability of a series system is obtained as the product of each device’s availability (0.9×0.9=0.81, 0.9×0.9×0.9=0.729). The more devices are added in series, the lower the overall availability becomes.
- Comparison against a baseline is judged by division. Three devices ÷ two devices = 0.729÷0.81=0.9, so this reads as 90% of the baseline, i.e. a 10% decrease.
用語