The focus of this lesson will be identifying the required operation (addition or subtraction) and developing students' use of a number line to solve addition and subtraction problems. Students solve addition and subtraction problems based on books being returned to, and checked out. Familiarity with a number line and problem situations that require addition or subtraction. Compose/decompose three-digit numbers using place value of hundreds, tens, ones. Vocabulary: place value, digit, hundreds, tens, ones, compose, decompose.