If a number is divisible by two co-prime numbers,

it is divisible their product

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Example
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80 is divisible by 4 & 5

Since 4 & 5 are co-prime,

80 will be divisible by 4 × 5 = 20 also

RoughFactors of 4 are 1, 2, 4

Factors of 5 are 1, 5

Since common factor is 1

∴ 4 & 5 are co-prime

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