120 minutes comes from the mistake of treating both HDDs as processed at the faster 400 MB/min. The desktop gives 8000 ÷ 400 = 20 minutes and the LCD gives 40000 ÷ 400 = 100 minutes, for a total of 120 minutes, but the desktop PC's speed is 200 MB/min.
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正解The time required is found by “capacity ÷ processing speed”. Calculate with 1 GB = 1000 MB. The desktop PC processes its 8 GB HDD at 200 MB/min, so 8000 ÷ 200 = 40 minutes. The LCD all-in-one PC processes its 40 GB HDD at 400 MB/min, so 40000 ÷ 400 = 100 minutes. The total for the 2 units is 40 + 100 = 140 minutes, so b) is correct.
×Incorrectウ
220 minutes comes from swapping the two units' processing speeds. Applying 400 MB to the desktop (8000 ÷ 400 = 20 minutes) and 200 MB to the LCD (40000 ÷ 200 = 200 minutes) gives a total of 220 minutes, but the speed assignment is reversed.
×Incorrectエ
240 minutes comes from the mistake of treating both HDDs as processed at the slower 200 MB/min. The desktop gives 8000 ÷ 200 = 40 minutes and the LCD gives 40000 ÷ 200 = 200 minutes, for a total of 240 minutes, but the LCD all-in-one PC's speed is 400 MB/min.
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Use the unit conversion for storage capacity (1 GB = 1000 MB) together with the relationship “time required = capacity ÷ processing speed”.
Assign each HDD its own correct processing speed, compute each time separately, and then add them up. The key point is not to mix up the speeds or treat all units as having the same speed.