※ Our check while writing this explanation could not fully confirm the published answer; scoring follows IPA's official key.
×Incorrectア
1 month is the value you get if you mistake the monthly backups for full backups instead of incremental backups. If all files were stored every month, the usage would pile up as 2,400 MB → 2,800 MB → 3,200 MB …, and by 2 months later (8,400 MB in total) it would exceed 8 GB, so it would only reach 1 month later. Item (2) of the [Backup Procedure] in this question specifies an incremental backup, so the monthly usage stays constant at 400 MB.
×Incorrectイ
4 months is the value you get if you confuse an incremental backup with a differential backup. With a differential backup, every run stores “all files added since the last full backup”, so the usage grows as 400 MB, 800 MB, 1,200 MB …, and from 2,400 + 200n(n+1) ≦ 8,000 you get n = 4. This question uses an incremental backup, which stores only what has been added since the immediately preceding backup, so this calculation does not apply.
○ウ
正解First work out, in megabytes, how much image data was recovered and how much is added each month, then see how many times that fits into 8 GB. From the premises shared by this group of questions (Group B), one image file is 2 MB, 200 files are added each month, and at the time of the accidental deletion the current PC held 2,000 files. 60% of those remained on the old PC, so the recovered files number 2,000 × 0.6 = 1,200, which is 1,200 × 2 MB = 2,400 MB. These are first stored as a full backup, so the USB drive uses 2,400 MB. After that, incremental backups are used, so each month only the files added since the previous backup are stored: 200 files × 2 MB = 400 MB, a constant amount. Subtracting the full backup from the USB drive capacity of 8 GB = 8,000 MB leaves 8,000 − 2,400 = 5,600 MB, and 5,600 ÷ 400 = 14, so 14 incremental backups fit. Backups can therefore be made up to the one 14 months after the full backup, and the correct answer is “ウ”.
×Incorrectエ
20 months is the value you get by forgetting the 2,400 MB taken up by the initial full backup and assuming that all 8,000 MB can be used for incremental backups, giving 8,000 ÷ 400 = 20. The full backup is also stored in a folder on the USB drive, so you have to subtract it before working out how many incremental backups fit.
ここがポイント
An incremental backup stores only “the files added or changed since the immediately preceding backup”, so the capacity used each time is constant (here 200 files × 2 MB = 400 MB). A differential backup, in contrast, stores “all changes since the last full backup”, so it takes more capacity with each successive run.
The basic way to estimate backup capacity is to first subtract the full backup from the total capacity, then divide the remainder by the size of one incremental backup. Convert to a byte amount using number of files × size per file, and do not forget the unit conversion between GB and MB (8 GB = 8,000 MB).