Answer: The number of plates needed is (C + B)/P
Explanation:
Ok, we have C cookies, B buns, and she wants to put P things in each plate.
The number of plates can be obtained by:
(C + B)/P = N
Where you divide the total number of things (cookies + buns) into the number P, that is the number that goes in each plate, and in this way, you find how many groups of P things there are, and that number of groups is equal to the number of plates needed.