This Problem Set is a part of the Lesson 1, Unit 4, Grade 2. In this lesson, students build upon their understanding of the base ten system and relate it to addition and subtraction. Using a sentence frame, they recognize that they must add and subtract like units and that the digit in the tens place changes when adding and subtracting 10. For example, 54 - 10 is 5 tens 4 ones minus 1 ten.