This Problem Set is a part of the Lesson 20, Unit 2, Grade 3. In this lesson, students estimate the differences of two numbers by rounding both subtrahends to either the nearest then or hundred, and then compare their estimates to the actual difference. For example, they estimate 362 - 189 by rounding to the nearest hundred (400 - 200 = 200) and the nearest ten (360 - 190 = 170). Then they calculate the actual sum and determine which estimating method was more accurate.