This Instructional Activity is a part of the Lesson 9, Unit 7, Grade 8. Students identify the size in error when truncating an infinite decimal to a finite number of decimal places. If you were given just the decimal expansions of numbers, without knowing which fractions produced them, do you think you could easily add the two decimals to find the decimal expansion of their sum? But these decimals are infinitely long, and there are no rightmost digits.