There is a total of 3 x 10 = 30 cookie slices. There are 24 + 1 + 1 = 26 people who ate the cookie pieces. There is 30 - 26 = 4 cookie slices left.
This justifies what answer for what question? Q & A:
Manny had 3 birthday cookie pies to share with his 24 classmates and his teacher, Mr. Keith. If each of the cookie pies were cut into 10 slices and Manny, his classmates, and Mr. Keith all had 1 piece, how many slices are left?
4