Number Sense - Addition and Subtraction

**Learning Goals:**- Solve problems involving the addition and subtraction of two-digit numbers, using a variety of mental strategies (e.g., to add 37 + 26, add the tens, add the ones, then combine the tens and ones, like this: 30 + 20 = 50, 7 + 6 = 13, 50 + 13 = 63)
- Add and subtract three-digit numbers, using concrete materials, student-generated algorithms, and standard algorithms.
- Use estimation when solving problems involving addition and subtraction, to help judge the reasonableness of a solution.