The greatest common multiple of 10 and 18 is infinite.
However, I suspect you want either:
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Multiples of 6 are 6, 12, 18, 24, 30, 36... Multiples of 10 are 10, 20, 30, 40... The LCM of 6 and 10 is 30. The greatest common multiple could go into infinity.
The greatest common factor is 2 and the least common multiple is also 2.
6
The greatest common multiple is infinite.
The Least Common Multiple of 10, 18, 21 is 630.