Mrs Kennedy is buying pencils for 315 students.
The pencils are sold in boxes of 10.
Therefore the number of pencils she needs to buy should be a multiple of 10, so that she can buy boxes of 10 pencils each.
number of students are 315 but 315 is not a multiple of 10
so we have to round off 315 to the nearest ten, that's 320.
so then she has to buy 320 pencils
number of boxes she needs - 320 / 10 = 32 boxes of pencils
she only needs pencils for 315 students so she will have 5 extra pencils
she will have to buy 32 boxes