In this unit, students will: • Explore, understand, and apply the commutative and associative properties as strategies for solving addition problems. • Share, discuss, and compare strategies as a class. • Connect counting on to solving subtraction problems. For the problem “15 – 7 = ?” they think about the number they have to count on from 7 to get to 15. • Work with sums and differences less than or equal to 20 using the numbers 0 to 20. • Identify and then apply a pattern or structure in mathematics. For example, pose a string of addition and subtraction problems involving the same three numbers chosen from the numbers 0 to 20, such as 4 + 13 = 17 and 13 + 4 = 17. • Analyze number patterns and create conjectures or guesses. • Choose other combinations of three numbers and explore to see if the patterns work for all numbers 0 to 20. • Understand that addition and subtraction are related and that subtraction can be used to solve problems where the addend is unknown. •Use the strategies of counting on and counting back to understand number relationships. • Organize and record results using tallies and tables. • Determine the initial and the change unknown.