PreK & Up. These clever pebbles are an ideal tool for building interest in numbers and math in a wide variety of contexts, including sand, water, and discovery play. This expanded set provides opportunities for beginning math operations. Includes two each of numbers 0-20, two each of the plus, subtract and equals operations, and one each of multiply and division operations. Each set includes a storage container and teacher's guide. Set of 50 pebbles. Pebbles are approximately 1 1/2" in size.