Distance Model - Subtracting

Many students come to understand subtracting as taking away. For example, \(10 - 3\) becomes known as "ten take away three." But, this is not the only context subtraction can represent. It can also be used to find the distance or difference of two numbers.

Screen Shot 2015-10-31 at 2.04.22 PMTake-away models often lead to lots of regrouping. To solve \(3000-199\), most students (and frankly adults) do a lot of regrouping.

This is a complicated procedure that is hard for many student to make sense of. Teachers often use base-10 blocks to model the regrouping (for example 1 thousand becomes 10 hundreds) but my experience has been that many students still struggle to apply and justify this algorithm even in older grades.

While it is important to know how to regroup, modeling subtraction as a distance can provide students with an alternative method for solving subtraction problems. So, what does modeling subtraction as a distance look like?

Distance Model for SubtractingWhen you think of the result of a subtraction problem as the distance (really it's a vector) between the two given numbers (minuend - subtrahend) it is easy to see that you can adjust both the minuend and subtrahend by the same value and the difference will not change.[7]For visual proof of this, see the above gif. As the left and right end of the line segment increase or decrease by the same amount, the length of the line does not change. Put more simply, you can increase both the numbers in a subtraction problem by the same amount and the difference remains the same.[8]Technically you are increasing the first number and subtracting more with the second number.

Algebraically, it looks like this:

Statements Reasons
\(a-b=a-b\) Given
\(a-b+0=a-b\) Additive Identity[9]Any number plus 0 equals the given number.
\(a-b+(1-1)=a-b\) \(0 = 1-1\)
\(a+1-b-1=a-b\) Commutative Property[10]By treating \(-1\) as \(+(-1)\) you can move subtraction operands around as well.
\((a+1)-(b+1)=a-b\) Factoring[11]\(-b-1=-(b+1) \) because I factored out a \(-1\). I simply grouped \((a+1)\) to make the result easier to see.

 

Being able to envision subtraction this way yields efficient ways of subtracting all sorts of numbers. Here is @bstockus demonstrating it on twitter for whole numbers:

This process works well for whole numbers, but it really shines with fractions. Consider for a moment the challenge of this subtraction problem:

\(3\frac{1}{8}-1\frac{3}{4}\)

Solving this requires making common denominators, then regrouping from minuend by turning one whole into eighths or by making an improper fraction.

Either \(2\frac{9}{8}-1\frac{6}{8}\) or \(\frac{25}{8}-1\frac{6}{8}\)

If instead I increase both the minuend and subtrahend by the same amount, I can turn this into a much easier problem; one that does not require borrowing.

\((3\frac{1}{8}+\frac{2}{8})-(1\frac{6}{8}+\frac{2}{8})\)

\(3\frac{3}{8}-2=1\frac{3}{8}\)

The same move can be accomplished with decimals too. In summary, this strategy is really handy any time it is easier to do a bit of adding (or subtracting) instead of subtraction with lots of regrouping.

Strengths

  • Supports student understanding of a lessor used interpretation of subtraction
  • Provides an alternative to regrouping in subtraction algorithms
  • Can be used with whole numbers, fractions, and decimals
  • Requires introducing students to distance model of subtraction

Weaknesses

  • Requires that students are familiar with distance model of subtraction
  • Not as efficient in all circumstances

Sources[12]Leave a note in the comments if you know of another source where this model appears.

  • Ben shares that EMP and EngageNY use this model (and he shares some other great thoughts in the comments).