If the last digit doubled subtracted from the rest is a multiple of 7, the whole number is divisible by 7.
or
Divide 7 into it. If the answer has no remainder, the number is divisible by 7.
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28 = 2 × 2 × 7 = 2² × 7.
As a product of its prime factors: 5*5*7 = 175
A number can be factored to determine whether it is prime or composite. A prime number has exactly 2 factors, 1 and the number itself. A composite number has more than two factors.
73 7*7*7 = 343
b2 + 8x + 7. 7 and 1 add up to be 8 and multiply to be 7. So therefore, it would be factored as: (b+1)(b+7)