Two-Digit Left To Right Mental Addition

All of us are taught to do maths addition operation from right to left.  That is OK for doing maths on paper.  But if we want to do it mentally, it is better to work from left to right.  After all, we pronounce numbers from left to right.

Consider this addition problem :

45 + 34

Most of us would not like to do this problem in our head.  But let’s simplify it .

To solve 45 + 34, first we add 30, then add 4.  After adding 30, we have a simpler problem 75 +4, which equals 79.  We illustrate this as follows :

45   +   34     =     75   +   4     =     79

             (first add 30)      (then add 4)  

The above diagram is simply a way representing the mental process involved in arriving at our answer.

The above problem problem does not require you to carry any numbers.  Now let’s try a calculation that requires you to carry a number :

58 + 37

Adding from left to right, we can simplify the problem by adding 58 + 30 = 88, then 88 + 7 = 95.

58    +    30      =      88    +    7      =      95

                (first add 30)           (then add 7)

OK, now try 68 + 57

68     +     57      =      118     +     7     =     125

                  (first add 50)             (then add 7)

If carrying numbers trips you up a bit, don’t worry about it because this is the first time you have ever made an attempt at mental calculation. You will take time to get used to it.  With practice, you will begin to see these numbers in your mind and carrying numbers when you add will come automatically.

Exercise :

1.   24  +  33                           

2.  54  +  23

3.  85  +  34

4.  26  +  64

5.  19  +  15 

Answers :

1.  57

2.  77

3.  119

4.  90

5.  34

Reference : Think Like A Maths Genius by Arthur Benjamin and Michael Shermer.

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Posted on April 26, 2009, in Info Matematik. Bookmark the permalink. Leave a comment.

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