# Divisibility by 7

**Tutor description**

Divisibility by 7: Now we will study divisibility by 7. One book on speed arithmetic says that these tests are just too complicated, and you should just divide by 7. I agree to some extent, but my calculator still will not let me enter really large numbers. One interesting way (found in some books) is to take the two left-most digits, multiply the left digit by 3 and add it to the second digit. Replace these two digits with

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