From the problem, only 22% of the students worked part-time after school
and there's a +-8% margin of error
So the percentage range who do part-time after school is :

The range of the number of students will be the total number of students multiplied by the percentage.
3420 x 0.14 = 478.8 or 478
3420 x 0.30 = 1026
So the range of the students who part time is :
From 478 to 1026
Since we are looking for the least possible number.
The answer is 478