I assume you're asking about #19?
24 students total, teacher gives each PAIR of students 3 magnets.
There are 12 pairs of students, then, and the teacher has given 9 of those pairs their 3 magnets, leaving 3 pairs without their magnets.
3 pairs remaining to get 3 magnets (3*3=9)
The teacher needs 9 more magnets.
