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.
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!