Mr. Sato needs 9 magnets.
Explanation:
AS there are 24 students in the science class,
there are <span><span>242</span>=12</span> pair of students.
Mr. Sato wants to give each pair of students 3 magnets
Mr. Sato has already given 9 pairs of students their 3 magnets
Therefore number of pairs of students yet to be given magnets is <span>12−9=3</span>.
For them he needs <span>3×3=9</span> magnets.