A set 'A' is defined to contain any positive integer which has the property of being a string of "21's", "42's", and/or "421's". If all the integers in set A are ordered from smallest to largest, what is the 21st term of this sequence?

For clarification, the sequence starts out: 21, 42, 421, 2121, etc.

The answer is 2121421.

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The sequence: (1) 21, (2) 42, (3) 421, (4) 2121, (5) 2142, (6) 4221, (7) 4242, (8) 21421, (9) 42121, (10) 42142, (11) 42421, (12) 212121, (13) 212142, (14) 214221, (15) 214242, (16) 421421, (17) 422121, (18) 422142, (19) 424221, (20) 424242, (21) $\boxed{2121421}$