Easy. Follow these steps:
1. Take the last digit of the number you’re testing and double it.
2. Subtract this number from the rest of the digits in the original number.
-If this new number is either 0 or if it’s a number that’s divisible by 7, then you know that the original number is also divisible by 7.
-If you can’t easily tell yet if the new number is divisible by 7, go back to the first step with this new smaller number and try again.
For example:
Is 345 divisible by 7?
Let’s use the three-step-process to find out.
1. Last digit: 5
double it: 10
2. Subtract 10 from the rest of the digits. 34-10=24
– this is not 0 not a number divisible by 7.
So 345 is not divisible by 7!
Let’s try another one: 175
1. Last digit 5
2. Double it:10
3. 17-10=7— IT’S DIVISIBLE BY 7!
Easy and fun? Try it yourself!