There are 24 students in a science class. Mr. Sato will give each pair of students 3 magenta. So far, Mr. Sato has given 9 pairs of students their 3 magnets. How many more magnets does Mr. Sato need so that each pair of students has exactly 3 magnets? Write down an expression.
9 pairs = 18
24 - 18 = 6 = 3 pairs