This Problem Set is a part of the Lesson 19, Unit 2, Grade 6. Students explore and discover that Euclid's algorithm is a more efficient means to finding the greatest common factor of larger numbers and determine that Euclid's algorithm is based on long division. Students look for and make use of structure, connecting long division to Euclid's algorithm. Students look for and express regularity in repeated calculations leading to finding the greatest common factor of a pair of numbers.