A third is exactly how it sounds: 1 piece out of 3 pieces of one number. If you imagine a cake is cut into 3 slices, one third would be one of those slices. This means that we have 3 thirds in any whole number. We can show this by multiplication: 3*(1/3) = 3/3 = 1. Therefore, if we have 9 whole numbers, and there are three thirds in every whole number, then there are 9x3 thirds in 9, which is 27.