The effective method of doing additions and subtractions in mental computation

Mental Computation is the ability to compute certain problems mentally especially in mathematics. It emphasizes the mental processes used to achieve the answer without the use of any computational aid. On the other hand, mental arithmetic focuses on speed and accuracy.

There is a greater emphasize on mental computation and understanding of number in the new Brunei Mathematics syllabus particularly in the teaching and learning of mathematics (Curriculum Development Department, 2007). The new syllabus was introduced in the year 2004. These mathematical skills would enable the students to cope with everyday life for instance calculating, estimating and solving problems.

Starting year 2008, all the primary 6 students will be assessed based on mental computation as part of their Penilaian Sekolah Rendah (PSR) examinations. It will contribute 10 percents of the final marks and comprises of 30 questions within 10 minutes time.

Easier methods should be introduced and taught to the students so that they can do additions and subtractions accurately without doing the traditional paper and pencil algorithms. This can develop their mental computations and also thinking skills.

Students used many techniques in doing mental computations. Some of them are count on method, count back method, choose compatible numbers methods, left to right method, compensation method, equal additions method, sequence method and split tens method.

Examples of strategies:

Sequence method

Addition
Question: 27 + 35,
Step 1: (27 + 30) + 5,
Step 2: 57 + 5,
Step 3: (57 + 3) + 2,
Step 4: 60 + 2,

Result: 62.

Subtraction

Question: 45 – 27,

Step 1: (45 – 20) – 7,

Step 2: (25 – 5) – 2,

Step 3: 20 – 2,

Result: 18.

In this method, the second number has to be sequence by separating the tens from the ones. In that way, it would be easier to add or to subtract the numbers step by step. Other than that, the students can use number bonds method or counting on method which they were familiar with since their lower primary level. This can help them to answer the questions easily.

Split tens method

Addition
Question: 27 + 35,
Step 1: (20 + 30) + (7 + 5),
Step 2: 50 + 12,
Step 3: (50 + 10) + 2,
Answer: 62.

Subtraction

Question: 45 – 27,

Step 1: (40 + 5) – (20 + 7),

Step 2: (30 + 15) – (20 + 7),

Step 3: (30 – 20) + (15 – 7),

Step 4: 10 + 8,

Answer: 18.

In this method, the numbers in the tens and the ones place value have to be split so that the students can focus in adding or subtracting the numbers in ten place value and the numbers in the ones place value. But if in subtraction (refer to the above example), in this case 5 cannot be subtracted with 7 so to solve the problem the number in the ten place value has to be decrease so that numbers can be subtracted (15 – 7).

Implications on the teaching and learning of Mathematics

These strategies will help the students to be more fascinated in learning mathematics. As they happen to know new ways of doing mental computation especially in additions and subtractions and they were also given the freedom to choose which method that they preferred. They would be happy if they are successful in doing new things. This was because students at this age are very curious. This will also encourage them to learn new strategies. High ability students could proliferate the methods introduced to them by teaching the methods to their other friends.

(These texts were taken from my PS 1234’s report, November 2008)