Number Problem 11

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If two numbers have a common factor, that factor will divide their sum and difference exactly.

True. Let x and y are two numbers and m be their common factor.
If x = am and y = bm,
we get x + y = am + bm = m(a + b) which is divisible by m
Similarly, x – y = am – bm = m(a - b) which is also divisible by m.