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You Can Divide by 7 If…
I know I haven’t written in a while. That’s because I had nothing new to show you. I mistakenly thought that I’d figured out all the tricks in basic arithmetic. I was wrong. I got two new tricks within a week of each other, so here’s this one and the other will be coming out as soon as I make the graphics for it.
I was getting my Dr. Karen’s Math Hacks: Basic Arithmetic book translated into Spanish recently. I had an AI do the basic translation and then had a native Spanish speaker (Carlos G on Fiverr) proofread and fix it. In that book, I said that I didn’t know of any way to tell if a number was divisible by 7, and that if you did, to let me know. Well, my proofreader knew of a trick, and I’m passing it on. So thanks, Carlos!
Here’s the trick
- Separate the 1s digit from the rest of the number, and double it.
- Subtract that from the rest of the number.
- If the remainder is 0 or divisible by 7, then the whole number is divisible by 7.
Example 1: Is 567 divisible by 7?
Step 1: Change 567 into 56 and 7, then double the 7 to get 14.
Step 2: Subtract 56–14 = 42
Step 3: Is 42 divisible by 7? Yes, 6 x 7 = 42
