Develop methods for addition and subtraction

Lessons include: Use knowledge of place value to add large numbers (up to 4 digits) mentally; Use knowledge of place value to subtract large numbers (up to 4 digits) mentally; Subtract mentally by finding the difference, targeting the ten, hundred or thousand; Estimate by rounding and check using the inverse operation with four-digit numbers; Add large numbers using column addition; Subtract large numbers using column subtraction; Exchanging across zero with column subtraction; Solve missing number problems for addition and subtraction with four-digit numbers