The Princess' Problem, also known as the 'PEA Under the Mattress Problem', is a classic problem in mathematics. It involves a princess who is trying to find a PEA hidden under a stack of mattresses. The problem is to determine the minimum number of mattresses that must be removed in order to find the PEA. The solution to this problem is the letter PEA, which stands for the mathematical concept of Permutations and Exhaustive Search. This concept states that all possible combinations of elements must be examined in order to find the desired result.