Dealing with Subtraction (III)


While teaching subtraction involving analysis of the numbers teachers might emphasize some of the following aspects as well:

Likeness in subtraction: In this case, if the child is solving an example of the type "62 - 37" the teacher may tell them that 2 and 7 both are numbers of ones, while the 6 and 3 both are numbers of tens. Repeating these facts helps the child understand the decomposition or equal addition of numbers.

Proper placement of digits and order of operation: The correct written form for "67 - 24" is the form
  The order of performing the subtraction does not influence the answer in these examples without decomposition, but the pattern of subtracting ones first, then tens and so on should be established, since it is a general approach applicable to all types of subtraction.

Correct verbalizing: This helps in fixing the meaning of what is being performed. Chlidren should state each subtraction in a manner indicating exactly what underlies the steps. Thus, the subtraction "67 - 24" is verbalized as " 4 ones from 7 ones are 3 ones; 2 tens from 6 tens are 4 tens; the answer is 4 tens and 3 ones, or 43."

The short-cut pattern: When the child fully understands the process of subtraction, teaching of the abridged method should follow. For the example given above, the child's thinking would be "four from seven, two from six" as s/he is performing the operation.

Let us now see how children could be taught to subtract mentally.

Subtracting Mentally

In the real situation we need to perform subtractions without using paper and pencil. There are many ways to accomplish it.

  1. Subtract the tens of the subtrahend from the minuend, then subtract ones. For example, "98 - 73" is subtracted as '(98 - 70) - 3' where the steps are 98 - 70 = 28 -3 = 25. Similarly, "64 - 36" is performed by the two subtractions: 64 - 30 = 34 and 34 - 6 = 28.
  2. Subtract the ones in the subtrahend from the minuend, then subtract the tens. For example, "98 - 73" is subtracted as (98 - 3) - 70 where the steps are: 98 - 3 = 95 and 95 - 70 = 25. Likewise, for 64 - 36 the result of the two consecutive subtractions are 58, then 28, where 58 is 64 - 6 and 28 is 58 -30.
  3. Application of compensation described in the previous article offers another method for performing subtractions mentally.
    The copyright of the article Dealing with Subtraction (III) in Math for Kids is owned by Vidya Narayan Wadadekar . Permission to republish Dealing with Subtraction (III) in print or online must be granted by the author in writing.

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