This divisibility criterion differs from all others previously studied (Divisibility by 2, Divisibility by 3, Divisibility by 5). In this criterion, it is necessary to repeat a process until you find a value that can be easily compared to the multiples of the number 7.
The process that must be done to verify the divisibility by 7 is as follows: "Multiply the last digit of the number by 2. Subtract this value from the initial number without the last digit, the result must be a multiple of 7.”
Note that this is not as simple a process as the previous ones, in which it was enough to analyze the last digit, or add the digits of the numbers. In this, the process will be repeated in order to reduce the number of digits to be analyzed for divisibility by 7. See an example carrying out the aforementioned process:
"Check the divisibility by 7 of the number 7203"
If you want, you can stop the process at the number 63, because in it we've already been able to determine whether the number is actually divisible by 7 or not. We know that 63 = 7x9, so 7203 can be divided by 7.
"Check the divisibility by 7 of number 6481"
Note that the number 52 is not a multiple of 7, so the number 6481 is not divisible by 7.
By Gabriel Alessandro de Oliveira
Graduated in Mathematics
Kids School Team
