# Section C: Primes/Divisibility

Since the concept of ‘common multiple’ is needed to teach children how to add and subtract fractions with unlike denominators, we include a short section that leads up to an explanation of the procedures for computing the greatest common factor and least common multiple of two whole numbers. A powerful and efficient technique for performing these computations uses prime factorizations, so we begin with a discussion of prime numbers. As the topic of primes can be very rich at the elementary level, I have included some slightly advanced results and examples as attachments for the interested teacher.