How to tell about the soroban for kids

complementary numbers

The idea of complementary numbers are crucial for the soroban culculations. Here are some methods for kids to memorize them.

We can easily find the complementary numbers by using five fingers of our hand. Form a fist and raise a finger from it. Now you will see that there are four fingers folded and one finger raised as shown in the figure below. From this you can obtain that the 1’s complementary number is 4. Likewise, 2, 3 and 4’s complementary numbers can also be obtained.

1’s complementary number to 5 is 4
2’s complementary number to 5 is 3
3’s complementary number to 5 is 2
4’s complementary number to 5 is 1

Some time especially teaching that to small children, we use another term, a friend of numbers. Instead of “1’s complementary number to 5 is 4” we explain “1’s friend to be 5 is 4”. Because this term, friend, is easy to memorize for children and expresses those two numbers strong relationship.

1’s friend to be 5 is 4
2’s friend to be 5 is 3
3’s friend to be 5 is 2
3’s friend to be 5 is 1

To obtain the complementary numbers to 10 we can use the same idea as the one to 5 using the fingers in our hand. The only difference is that we use both the hands in this case. If you bend nine fingers on both of your hands, you will see that a finger remain straight. This indicates that 9’s complementary number to 10 is 1. Similarly, you can also obtain the 8’s complementary number’s to10 as 2,
7’s complementary number’s to10 as 3
and 6’s complementary number’s to10 as 4.

9’s complementary number to 10 is 1
8’s complementary number to 10 is 2
7’s complementary number to 10 is 3
6’s complementary number to 10 is 4