Subtraction Fluency

Subtraction Facts

Like in addition, any subtraction can be reduced to a series of “simple” subtractions, each subtracting a single digit number from a single or double digit number.

In each of these “simple” subtractions, the number “to be subtracted” will always be a single digit number. The number “subtracted from” can either be a single digit number or a double digit number. “Subtracting from” a double digit number will occur only if “1” has been “borrowed” from the next higher “place”. Hence this double digit number would be a number between “10 and “18”. (If the number is 9, then there would be not be any need to “borrow”).

The result of this subtraction will always be a single digit number

There are 3 procedures for doing subtraction – Counting the Remaining, Counting Backwards & Subtracting by Counting On.

Counting the Remaining (both numbers <=9)

This is the simplest case.

The bigger number is represented with extending fingers on one or both hands. As many fingers as the second number are folded down. The remaining “extended fingers” gives the result.

Counting Backwards

Subtraction can be done by representing one of the numbers with finger abacus and counting backwards (and folding the fingers to avoid confusion). But counting backwards is a difficult skill for many children.

Some students may find it easy. It should not be insisted upon for all students.

Counting On

This idea is based on the“ How Much/Many More?” metaphor.

With this procedure, all possible subtraction facts can be worked out. Let us take 15-9.

For doing 15-9, 9 is remembered -> 10 (1) -> 11 (2) -> 12 (3) -> 13 (4) -> 14 (5) -> 15 (6). The numbers in the bracket indicate the number of fingers which are extended. When the “counting on” reaches 15, 6 fingers are extended. The result is 6.

Since the result of the subtraction will always be a single digit number, the number of fingers which need to be counted can always be accommodated with both hands.

Using the Ruler as a calculator – Number Line concept

Two rulers, each with cm markings up to 30, can be used by students as a calculator. For larger numbers the meter scale with markings up to 100 can be used. The procedure is a “inversion” of the addition procedure.

Let us take 23-9. Keep the rulers parallel, one above and numbers increasing from left to right. Keep the scales such that the 23 of the upper scale coincides with 9 of the lower scale. The result (14) can be read out from the reading on the upper scale which lies just above the 0 in the lower scale.

This method is a good introduction to the concept of the number line and using it for subtraction.

Achieving Fluency in Subtraction

Subtraction fluency can be built by combining addition fluency & the relations between addition & subtraction. Doing subtraction problems as “related addition” problems is actually a lot easier.

For example, 14 -8 can be thought of as “what plus 8 gives 14” and the answer is 6!