56 Addition and Subtraction ConceptsWhat is Comparison Subtraction?
Comparison subtraction is one of three types of subtraction. In
comparison subtraction, the focus is on the difference between
quantities. Comparing means fi nding out how many more or
how many less one number is compared to another number.
As with missing part subtraction, comparison subtraction
is not “take away.” Finding the difference does not involve
removal; therefore we should call it subtraction rather than
“take away.” Differences are reported in absolute values. This
means that we do not need to teach students rules such as the
big number goes fi rst when we subtract. Instead, we can focus on
teaching the concept of comparison subtraction because the
difference between two numbers can be positive or negative.CCSS
Operations and Algebraic Thinking
Number and Operations in Base TenFormative Assessment
To fi nd out if a student understands comparison subtraction,
give the student some numbers to compare. Ask the student
to explain how he solved each problem.What is the difference between 5 and 4?
What is the difference between 10 and 7?
What is the difference between 15 and 9?
What is the difference between 20 and 12?If the student is successful, try numbers with greater
differences. If the student is not successful, encourage the
student to use objects to model the differences.Comparison Subtraction...............................................................................
CONCEPT: