Answer: 12 weeks
Step-by-step explanation: if someone finishes 1 subject every 4 days, we can calculate the number of weeks it would take to finish 21 subjects.
Since they finish 1 subject every 4 days, we can divide 21 by 1/4 to find the total number of 4-day periods needed to finish 21 subjects.
21 / (1/4) = 21 * 4 = 84
Therefore, it would take approximately 84 days to finish 21 subjects. To convert this to weeks, we divide 84 by 7 (since there are 7 days in a week).
84 / 7 = 12
So, it would take approximately 12 weeks to finish 21 subjects.