If you double the last digit and subtract it from the rest of the number and the answer is zero or divisible by 7, then the whole number is divisible by 7.
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Since 81 is not divisible by 7 therefore, 7 is not a factor of 81.
Divide 7 into 291. If it comes out evenly, it's a factor. It doesn't, so it's not.
To find out if 7 is a factor of 2395, divide 7 into 2395 and you'll get 342 and a remainder of 1. This means 7 is not a factor of 2395.
The GCF of 7, 21, and 42 is 7, because 7 is a factor of all three.
1 is a factor of 7.