Wednesday, February 26, 2020

119 Divisibility test of 7

Dear friends,

When I was in 7th grade in 1972, my teacher taught me the divisibility tests of 2, 3, 4, 5, 6, 8, 9, 10,  except 7, then I asked her the divisibility test of 7. On this, she told that there is no test available for 7. I took it as a challenge.

While studying the table of 7 carefully, one day I fount the wonderful method for the divisibility test of 7. Actually, it's very interesting.

This method is published in my book titled "JADU NAGARI SANKHYANCHI". (Magic world of numbers) written in Marathi by "Bruhan Mumbai Ganit Adhyapak Mandal, Mumbai" published on 26th Dec 1980.

Procedure:

1) Take a number.
2) Write down its unit placed digit.
3) Double this unit placed digit.
4) Write down our new number excluding unit placed digit.
5) Subtract double of the unit placed digit from our new number.
6) Repeat this till we get a 2 or 1 digit number. If this number is divisible by 7, then our original number is also divisible by 7.

Example-1:


Check the divisibility of 7 for the number:
6902
1
Your Number
A
6902
6902
2
Its Unit Placed Digit
B
2
2
3
Double of Unit placed Digit
2B
2x2
4
4
Your new Number excluding Unit Placed Digit
C
690
690
5
Subtract double of unit placed digit from remaining number
C-2B
690-4
686
Repeat the same procedure
6
Your Number
A
686
686
7
Its Unit Placed Digit
B
6
6
8
Double of Unit placed Digit
2B
2x6
12
9
Your new Number excluding Unit Placed Digit
C
68
68
10
Subtract double of unit placed digit from remaining number
C-2B
68-12
56
As 56 is divisible by 7, our number 6902 is also divisible by 7.

Example-2:


Check the divisibility of 7 for the number:
1981
1
Your Number
A
1981
1981
2
Its Unit Placed Digit
B
1
1
3
Double of Unit placed Digit
2B
2x1
2
4
Your new Number excluding Unit Placed Digit
C
198
198
5
Subtract double of unit placed digit from remaining number
C-2B
198-2
196
Repeat the same procedure
6
Your Number
A
196
196
7
Its Unit Placed Digit
B
6
6
8
Double of Unit placed Digit
2B
2x6
12
9
Your new Number excluding Unit Placed Digit
C
19
19
10
Subtract double of unit placed digit from remaining number
C-2B
19-12
7
As 7 is divisible by 7, our number 1981 is also divisible by 7.

Example-3:


Check the divisibility of 7 for the number:
20237
1
Your Number
A
20237
20237
2
Its Unit Placed Digit
B
7
7
3
Double of Unit placed Digit
2B
2x7
14
4
Your new Number excluding Unit Placed Digit
C
2023
2023
5
Subtract double of unit placed digit from remaining number
C-2B
2023-14
2009
Repeat the same procedure
6
Your Number
A
2009
2009
7
Its Unit Placed Digit
B
9
9
8
Double of Unit placed Digit
2B
2x9
18
9
Your new Number excluding Unit Placed Digit
C
200
200
10
Subtract double of unit placed digit from remaining number
C-2B
200-18
182
Repeat the same procedure
11
Your Number
A
182
182
12
Its Unit Placed Digit
B
2
2
13
Double of Unit placed Digit
2B
2x2
4
14
Your new Number excluding Unit Placed Digit
C
18
18
15
Subtract double of unit placed digit from remaining number
C-2B
18-4
14
As 14 is divisible by 7, our number 20237 is also divisible by 7.

In the same way, we can check the divisibility test for any number.

ANIL SATPUTE

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