1/8 is merely the reciprocal of the 8 hours of daily working time, and it does not use Mr. A's working time of 36 hours at all. The 8 is the number you multiply by (the working hours in one day), not the number you divide by. The amount that can be completed in one day is found by multiplying 1/36 by 8, so putting 8 in the denominator reverses the operation.
×Incorrectイ
5/36 comes from multiplying the hourly amount 1/36 by the 5 working days instead of by the 8 working hours per day (so it is only the amount Mr. A finishes in 5 hours). The unit of 1/36 is per hour, so what you should multiply by is the number of hours in a day, 8, not the number of days, 5. Always make sure the units add up.
×Incorrectウ
1/6 is the same as 6/36, the value obtained by multiplying 1/36 by 6, that is, the answer you get if you mistake the daily working time for 6 hours instead of 8 hours. The passage clearly states that the working time per day is 8 hours, so the number 6 has no basis anywhere in the question.
○エ
正解The correct answer is エ, 2/9. First, take the reference quantity to be the workload of one week of mail-order operations and set it to 1. From the table, Mr. A finishes this in 36 hours, so the amount Mr. A can complete in 1 hour is 1/36 of it (the passage states this explicitly). Next, to find the amount he can complete in one day, just multiply by the number of hours worked per day. The passage says Company F's working time per day is 8 hours, so 1/36 × 8 = 8/36 = 2/9. Do not forget to reduce the fraction: turning 8/36 into 2/9 makes it match the choice. As an alternative, you can reason that 36 hours ÷ 8 hours/day = 4.5 days to finish one week's workload, so one day's amount is 1 ÷ 4.5 = 2/9 of the whole.
ここがポイント
The work-rate way of thinking that sets the whole job to 1: if one week's workload is 1, then a person who finishes it in t hours processes 1/t per hour. To get the amount per day from this, multiply by the working hours in one day (1/36 × 8 = 2/9).
Build the habit of checking that the units are consistent: 1/36 is a per-hour amount, so what you multiply by is the number of hours (8 hours), not the number of days (5 days). Reduce the fraction you obtain so that it matches the form of the choices (8/36 → 2/9).