List the prime factors of each.
18 = 2 x 3 x 3,
42 = 2 x 3 x 7
Common factors are 2 and 3 so HCF = 6.
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The HCF of both numbers is 6
hcf = 42, lcm = 84.
The GCF is 42.
The HCF is 7.
gcf(18, 21, 42) = 3. 18 = 2 x 3^2 21 = 3 x 7 24 = 2^3 x 3 hcf = 3