The little-known divisibility test for 7 is

Take 91 as the number, Separate the units digit from the rest of the number, then multiply that units digit by 2. Subtract that from what’s left of the original number. If the result is a multiple of 7, the original number is a multiple of 7. Here’s how that works for 91. Separate 91 into 9 and 1.

Multiply 1 by 2: 1 X 2 = 2. Subtract: 9 – 2 = 7. 7 is obviously a multiple of 7, so 91 is a multiple of 7. Try it out on other multiples of 7, and you’ll see that it works every time...