He will be needing 4 packets of Graphing paper.
There are 28 students, each needed 3 sheets
28*3= 84sheets are needed in the class.
3 packets= 75 sheets, short for 9 sheets.
4 packets= 100 sheets.
Therefore, 4 packets of graphing paper are needed by the class. If it is possible to by 3 and 1/2 packets then it is enough.
Hope I help you. :) God Bless!