8.
300 ends in 0 or 5, so 5 is a factor
300 is divisible by 4 and 5, so 20 is a factor
300 is even and 3 + 0 + 0 = 3, so 6 is a factor
Leaving 8 to be the non factor
Testing if 8 is a factor: 4 x 3 + 2 x 0 + 0 = 12; 4 x 0 + 2 x 1 + 2 = 4 which is not 8, so 300 is not divisible by 8.
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5 is a prime factor of both numbers.
5, 10, 15 and 20 have five as a factor. The number 16 has five factors.
factor 12=2*2*3 20=2*2*5 25=5*5 LCM=2*2*3*5*5=300
Since 5 is a factor of 20, all the factors of 5 are common to 20.
The common factor of 2, 5 and 10 is 1.