14, 21, 28, 35, 42, 49, 56, 63, 70, 77 +7 . . .
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Divide 7 into 291. If it comes out evenly, it's a factor. It doesn't, so it's not.
Divide the number by thirteen. If the answer comes out even with no remainder, the number is divisible by thirteen.
Not if you divide correctly.
Divide a number and its factors by prime numbers until all the factors are prime. 210 Divide by two. 105,2 Divide by three. 35,3,2 Divide by five. 7,5,3,2 Stop. 2 x 3 x 5 x 7 = 210
An infinite number, starting with these: 42, 84, 126, 168, 210, 252, 294, +42 . . .