If a number is divisible by two co-prime numbers,
it is divisible their product
Example
80 is divisible by 4 & 5
Since 4 & 5 are co-prime,
80 will be divisible by 4 × 5 = 20 also
Rough
Factors of 4 are 1, 2, 4
Factors of 5 are 1, 5
Since common factor is 1
∴ 4 & 5 are co-prime